diff --git a/jest.config.js b/jest.config.js
index a63ca1c1..090cf8fc 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -14,6 +14,8 @@ export default {
     '/node_modules/(?!(color|color-string|color-convert|color-name)/)',
   ],
   moduleNameMapper: {
+    '\\.png$': '<rootDir>/src/test/mocks/assetStub.ts',
+    '\\?inline$': '<rootDir>/src/test/mocks/inlineStub.ts',
     '^@/(.*)$': '<rootDir>/src/$1',
     '^color$': '<rootDir>/src/test/mocks/color.ts',
     '^webextension-polyfill$': '<rootDir>/src/test/mocks/webextension-polyfill.ts',
diff --git a/src/SEQTA.ts b/src/SEQTA.ts
index af42696b..142554b2 100644
--- a/src/SEQTA.ts
+++ b/src/SEQTA.ts
@@ -53,6 +53,10 @@ if (document.childNodes[1]) {
   init();
 }
 
+// The 404 page is a bare document with no second childNode, so it must be
+// booted unconditionally (outside the gate above). It self-guards on title.
+bootErrorPage();
+
 if (import.meta.env.DEV) {
   window.addEventListener("unhandledrejection", (event) => {
     recoverFromStaleDevModuleGraph(event.reason);
@@ -138,6 +142,65 @@ async function init() {
   }
 }
 
+// The 404 page is a standalone document that never passes the SPA gate above,
+// so the normal plugin path never boots there; start just the classic kitten
+// 404 plugin so it renders without any SPA work.
+async function bootErrorPage() {
+  if (IsSEQTAPage) return;
+
+  // Runs at document_start, so <title> and body are not available yet; wait
+  // for the DOM before testing the 404 title.
+  await new Promise<void>((resolve) => {
+    if (document.readyState !== "loading") {
+      resolve();
+      return;
+    }
+    document.addEventListener("DOMContentLoaded", () => resolve(), {
+      once: true,
+    });
+  });
+
+  const is404Page =
+    /404/.test(document.title) || /not found/i.test(document.title);
+  if (!is404Page) return;
+
+  const stored = await browser.storage.local.get([
+    "onoff",
+    "plugin.error-page-kitten.settings",
+  ]);
+  if ((stored.onoff ?? true) === false) return;
+  const kittenEnabled = (
+    stored["plugin.error-page-kitten.settings"] as
+      | { enabled?: boolean }
+      | undefined
+  )?.enabled;
+  if (kittenEnabled === false) return;
+
+  hideRebranded404();
+
+  try {
+    const { pluginManager } = await import("@/plugins/index");
+    await pluginManager.startPlugin("error-page-kitten");
+  } catch (error) {
+    // Restore the rebranded page so the user is not left with a blank document.
+    document
+      .querySelectorAll<HTMLElement>(".bsplus-kitten-404-hidden")
+      .forEach((el) => {
+        el.classList.remove("bsplus-kitten-404-hidden");
+        el.style.display = "";
+      });
+    console.error("[BetterSEQTA+] Failed to boot 404 page:", error);
+  }
+}
+
+function hideRebranded404() {
+  const message = document.querySelector<HTMLElement>(".message");
+  if (message) {
+    message.classList.add("bsplus-kitten-404-hidden");
+    message.style.display = "none";
+  }
+}
+
 function replaceIcons() {
   document
     .querySelectorAll<HTMLLinkElement>('link[rel*="icon"]')
diff --git a/src/manifests/manifest.json b/src/manifests/manifest.json
index 82970383..095e37fb 100644
--- a/src/manifests/manifest.json
+++ b/src/manifests/manifest.json
@@ -43,6 +43,7 @@
   "web_accessible_resources": [
     {
       "resources": [
+        "resources/error-page/*",
         "resources/icons/*",
         "resources/update-image.webp",
         "resources/pdfjs/pdf.worker.min.mjs",
diff --git a/src/plugins/built-in/errorPageKitten/index.test.ts b/src/plugins/built-in/errorPageKitten/index.test.ts
new file mode 100644
index 00000000..655a0fbc
--- /dev/null
+++ b/src/plugins/built-in/errorPageKitten/index.test.ts
@@ -0,0 +1,115 @@
+/**
+ * @jest-environment jsdom
+ */
+/// <reference types="jest" />
+import type { PluginAPI } from "@/plugins/core/types";
+import errorPageKittenPlugin from "./index";
+
+const REBRANDED_404_BODY = `
+  <div class="message">
+    <h1>Page not found</h1>
+    <p>We can't find the page you're looking for.</p>
+    <p>It may have been moved, deleted or the link might be out of date.</p>
+    <p class="error-ref">Ref: 404</p>
+  </div>
+`;
+
+const api = {
+  settings: { loaded: Promise.resolve() },
+} as unknown as PluginAPI<{}>;
+
+async function startPlugin(): Promise<() => void> {
+  const cleanup = (await errorPageKittenPlugin.run(api)) ?? (() => {});
+  return cleanup;
+}
+
+function getCard(): HTMLElement | null {
+  // The card container shares its class with <html>, so scope to body.
+  return document.body.querySelector<HTMLElement>(".bsplus-kitten-404");
+}
+
+function expectKittenRendered(): void {
+  const card = getCard();
+  expect(card).not.toBeNull();
+  expect(document.documentElement.classList).toContain("bsplus-kitten-404");
+  expect(card?.querySelector("h1")?.textContent).toBe("404 Not Found");
+  // The original document's title.
+  expect(document.title).toBe("404 Not Found");
+  // The reference page's exact vendor-prefixed banner CSS is shipped at runtime.
+  const stripeStyle = Array.from(
+    document.head.querySelectorAll("style"),
+  ).find((s) => s.textContent?.includes("-webkit-repeating-linear-gradient"));
+  expect(stripeStyle?.textContent).toContain(
+    "repeating-linear-gradient(45deg",
+  );
+  const img = card?.querySelector(".kitten img") as HTMLImageElement | null;
+  expect(img?.getAttribute("alt")).toBeNull();
+  expect(img?.src).toContain("kitten");
+  const flickrLink = card?.querySelector(
+    ".attrib a",
+  ) as HTMLAnchorElement | null;
+  expect(flickrLink?.textContent).toBe("by storyvillegirl");
+  expect(flickrLink?.href).toBe(
+    "http://www.flickr.com/photos/bibbit/2756165489/",
+  );
+  const seqtaLink = card?.querySelector("a[href='http://www.seqta.com.au']");
+  expect(seqtaLink?.textContent).toBe("SEQTA");
+  // Original rebranded message is hidden.
+  expect(
+    document.querySelector(".message")?.classList.contains(
+      "bsplus-kitten-404-hidden",
+    ),
+  ).toBe(true);
+}
+
+describe("errorPageKitten", () => {
+  beforeEach(() => {
+    document.body.innerHTML = "";
+    document.title = "";
+    document.documentElement.classList.remove("bsplus-kitten-404");
+  });
+
+  it("renders the classic kitten 404 card over the rebranded page", async () => {
+    document.title = "Page not found";
+    document.body.innerHTML = REBRANDED_404_BODY;
+
+    const cleanup = await startPlugin();
+
+    expectKittenRendered();
+
+    cleanup();
+    expect(getCard()).toBeNull();
+    expect(document.title).toBe("Page not found");
+    expect(
+      Array.from(document.head.querySelectorAll("style")).some((s) =>
+        s.textContent?.includes("-webkit-repeating-linear-gradient"),
+      ),
+    ).toBe(false);
+    expect(
+      document.documentElement.classList.contains("bsplus-kitten-404"),
+    ).toBe(false);
+    expect(
+      document
+        .querySelector(".message")
+        ?.classList.contains("bsplus-kitten-404-hidden"),
+    ).toBe(false);
+  });
+
+  it("does not render inside the SPA (no .message, has #container)", async () => {
+    document.title = "SEQTA";
+    document.body.innerHTML = '<div id="container"></div>';
+
+    await startPlugin();
+
+    expect(getCard()).toBeNull();
+  });
+
+  it("does not render on an unrelated page", async () => {
+    document.title = "Home";
+    document.body.innerHTML = '<div id="app"></div>';
+
+    await startPlugin();
+
+    expect(getCard()).toBeNull();
+  });
+});
diff --git a/src/plugins/built-in/errorPageKitten/index.ts b/src/plugins/built-in/errorPageKitten/index.ts
new file mode 100644
index 00000000..c2731eb5
--- /dev/null
+++ b/src/plugins/built-in/errorPageKitten/index.ts
@@ -0,0 +1,132 @@
+import type { Plugin } from "@/plugins/core/types";
+import { resolveExtensionAssetUrl } from "@/lib/extensionAssetUrl";
+import kittenPng from "@/resources/error-page/kitten.png";
+import styles from "./styles.css?inline";
+
+const KITTEN_IMG = resolveExtensionAssetUrl(kittenPng);
+const KITTEN_CLASS = "bsplus-kitten-404";
+
+// The reference page's exact h1 background declarations, in the original
+// order. Each engine keeps the last declaration it understands (-webkit in
+// Blink, -moz in Gecko, the legacy standard otherwise), so shipping them
+// byte-for-byte reproduces the reference banner on every engine. Lives in a
+// JS string because the CSS minifier drops vendor-prefixed declarations.
+const H1_STRIPE_CSS =
+  "html.bsplus-kitten-404 .bsplus-kitten-404>h1{background-image:-moz-repeating-linear-gradient(135deg, rgba(0,0,0,0), rgba(0,0,0,0) 8px, rgba(0,0,0,0.05) 8px, rgba(0,0,0,0.05) 16px), -moz-linear-gradient(top, rgba(0,0,0,0), rgba(0,0,0,0.1));background-image:-webkit-repeating-linear-gradient(135deg, rgba(0,0,0,0), rgba(0,0,0,0) 8px, rgba(0,0,0,0.05) 8px, rgba(0,0,0,0.05) 16px), -webkit-linear-gradient(top, rgba(0,0,0,0), rgba(0,0,0,0.1));background-image:-o-repeating-linear-gradient(135deg, rgba(0,0,0,0), rgba(0,0,0,0) 8px, rgba(0,0,0,0.05) 8px, rgba(0,0,0,0.05) 16px), -o-linear-gradient(top, rgba(0,0,0,0), rgba(0,0,0,0.1));background-image:repeating-linear-gradient(45deg, rgba(0,0,0,0), rgba(0,0,0,0) 8px, rgba(0,0,0,0.05) 8px, rgba(0,0,0,0.05) 16px), linear-gradient(top, rgba(0,0,0,0), rgba(0,0,0,0.1))}";
+
+// The 404 page (legacy and rebranded) is a standalone document built around a
+// single `.message` block; the real app mounts a `#container`. Require the
+// former and rule out the latter so the plugin never renders inside the app.
+function is404Page(): boolean {
+  if (document.getElementById("container")) return false;
+  const heading = document.querySelector(".message > h1");
+  const text = heading?.textContent ?? document.title;
+  return /not found/i.test(text) || /404/.test(document.title);
+}
+
+function buildCard(): HTMLElement {
+  const card = document.createElement("div");
+  card.className = KITTEN_CLASS;
+
+  const h1 = document.createElement("h1");
+  h1.textContent = "404 Not Found";
+  card.appendChild(h1);
+
+  const p1 = document.createElement("p");
+  p1.textContent =
+    "Sorry — the resource you are looking for could not be found.";
+  card.appendChild(p1);
+
+  const p2 = document.createElement("p");
+  p2.textContent =
+    "If you believe you are seeing this message in error, please contact your IT department.";
+  card.appendChild(p2);
+
+  const kitten = document.createElement("div");
+  kitten.className = "kitten";
+
+  const kittenLine = document.createElement("p");
+  kittenLine.textContent = "I did find this picture of a kitten for you, though!";
+  kitten.appendChild(kittenLine);
+
+  const img = document.createElement("img");
+  img.src = KITTEN_IMG;
+  kitten.appendChild(img);
+
+  const attrib = document.createElement("div");
+  attrib.className = "attrib";
+  attrib.append("CC-BY-SA ");
+  const link = document.createElement("a");
+  link.href = "http://www.flickr.com/photos/bibbit/2756165489/";
+  link.textContent = "by storyvillegirl";
+  attrib.appendChild(link);
+  kitten.appendChild(attrib);
+
+  card.appendChild(kitten);
+
+  const seqtaLink = document.createElement("a");
+  seqtaLink.href = "http://www.seqta.com.au";
+  seqtaLink.textContent = "SEQTA";
+  card.appendChild(seqtaLink);
+
+  return card;
+}
+
+const errorPageKittenPlugin: Plugin = {
+  id: "error-page-kitten",
+  name: "Classic 404 Page",
+  description: "Brings back SEQTA's old kitten 404 page",
+  version: "1.0.0",
+  settings: {},
+  disableToggle: true,
+  defaultEnabled: true,
+  styles,
+
+  run: async (_api) => {
+    let card: HTMLElement | null = null;
+    let hasBuilt = false;
+    let originalTitle = document.title;
+    let stripeStyle: HTMLStyleElement | null = null;
+
+    const render = () => {
+      if (!is404Page()) return;
+
+      document.querySelector(".message")?.classList.add("bsplus-kitten-404-hidden");
+
+      if (hasBuilt) return;
+      const body = document.body;
+      if (!body) return;
+
+      hasBuilt = true;
+      document.documentElement.classList.add(KITTEN_CLASS);
+      document.title = "404 Not Found";
+      // Appended after the manager-injected styles so these same-specificity
+      // declarations win over the compiled baseline.
+      stripeStyle = document.createElement("style");
+      stripeStyle.textContent = H1_STRIPE_CSS;
+      document.head.appendChild(stripeStyle);
+      card = buildCard();
+      body.appendChild(card);
+    };
+
+    // Body may not be parsed yet at document_start; render now and again on
+    // DOM ready as a safety net.
+    render();
+    document.addEventListener("DOMContentLoaded", render, { once: true });
+
+    return () => {
+      document.removeEventListener("DOMContentLoaded", render);
+      document.documentElement.classList.remove(KITTEN_CLASS);
+      document
+        .querySelectorAll(".bsplus-kitten-404-hidden")
+        .forEach((el) => el.classList.remove("bsplus-kitten-404-hidden"));
+      document.title = originalTitle;
+      stripeStyle?.remove();
+      stripeStyle = null;
+      card?.remove();
+      card = null;
+    };
+  },
+};
+
+export default errorPageKittenPlugin;
diff --git a/src/plugins/built-in/errorPageKitten/lazy.ts b/src/plugins/built-in/errorPageKitten/lazy.ts
new file mode 100644
index 00000000..3bbe04a5
--- /dev/null
+++ b/src/plugins/built-in/errorPageKitten/lazy.ts
@@ -0,0 +1,14 @@
+import { defineLazyPlugin } from "../../core/dynamicLoader";
+import styles from "./styles.css?inline";
+
+export default defineLazyPlugin({
+  id: "error-page-kitten",
+  name: "Classic 404 Page",
+  description: "Brings back SEQTA's old kitten 404 page",
+  version: "1.0.0",
+  settings: {},
+  disableToggle: true,
+  defaultEnabled: true,
+  styles,
+  loader: () => import("./index"),
+});
diff --git a/src/plugins/built-in/errorPageKitten/styles.css b/src/plugins/built-in/errorPageKitten/styles.css
new file mode 100644
index 00000000..653c2dcd
--- /dev/null
+++ b/src/plugins/built-in/errorPageKitten/styles.css
@@ -0,0 +1,113 @@
+/* Classic SEQTA 404 page, restored one-to-one over the rebranded document.
+   Scoped under html.bsplus-kitten-404 so it can never leak into the SPA;
+   the card element itself carries the .bsplus-kitten-404 class. */
+
+/* The rebranded page styles <body> (Arial / flex / 25vh top / line-height
+   1.5 / letter-spacing). line-height and letter-spacing are reset because
+   the rebranded body sets them and the classic page uses browser defaults. */
+html.bsplus-kitten-404 {
+  background: #333;
+}
+html.bsplus-kitten-404 body {
+  background: #333;
+  font-family: 'Trebuchet MS', sans-serif;
+  font-size: 12pt;
+  color: #aaa;
+  margin: 8px;
+  padding: 0;
+  display: block;
+  line-height: normal;
+  letter-spacing: normal;
+}
+
+/* Hide the rebranded page content while the classic card is shown. */
+html.bsplus-kitten-404 .bsplus-kitten-404-hidden {
+  display: none !important;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 {
+  width: 600px;
+  margin: 64px auto;
+  background: #222;
+  border-radius: 4px;
+  box-shadow: -1px -1px rgba(0, 0, 0, 0.5),
+    1px 1px rgba(255, 255, 255, 0.2);
+  padding: 0 0 8px;
+  text-align: center;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 > h1 {
+  background: #612;
+  /* Valid modern baseline; the reference's exact vendor-prefixed declarations
+     are injected at runtime (H1_STRIPE_CSS in index.ts), which win over this.
+     The vendor-prefixed lines can't live in this file because the CSS
+     minifier drops them. */
+  background-image: repeating-linear-gradient(
+      135deg,
+      rgba(0, 0, 0, 0),
+      rgba(0, 0, 0, 0) 8px,
+      rgba(0, 0, 0, 0.05) 8px,
+      rgba(0, 0, 0, 0.05) 16px
+    ),
+    linear-gradient(to bottom, rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.1));
+  color: #fff;
+  padding: 8px;
+  border-radius: 4px 4px 0 0;
+  text-shadow: 0 1px rgba(0, 0, 0, 0.5);
+  font-size: 150%;
+  font-weight: normal;
+  margin: 0 0 16px;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 > p {
+  padding: 8px;
+  margin: 0;
+  text-shadow: 0 1px rgba(0, 0, 0, 0.5);
+  font-style: italic;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 > p > a {
+  color: #729fcf;
+  text-decoration: none;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 > .kitten {
+  font-size: 70%;
+  padding: 24px 0;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 > .kitten > p {
+  color: #666;
+  font-style: italic;
+  margin: 0 0 8px;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 > .kitten > .attrib {
+  color: #333;
+  font-size: 80%;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 > .kitten > .attrib > a {
+  color: inherit;
+  text-decoration: none;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 > .kitten > img {
+  display: inline-block;
+  border-radius: 4px;
+  box-shadow: 0 0 4px #000;
+}
+
+html.bsplus-kitten-404 .bsplus-kitten-404 > a {
+  display: block;
+  padding: 8px;
+  margin: 0 8px 0 auto;
+  width: 100px;
+  text-align: center;
+  color: #444;
+  background: #111;
+  text-decoration: none;
+  font-size: 80%;
+  border-radius: 4px;
+  box-shadow: inset 1px 1px rgba(0, 0, 0, 0.5);
+}
diff --git a/src/plugins/core/manager.ts b/src/plugins/core/manager.ts
index 0719975f..6a1ec151 100644
--- a/src/plugins/core/manager.ts
+++ b/src/plugins/core/manager.ts
@@ -26,6 +26,9 @@ interface StorageChange<T = any> {
 
 /** Phased plugin startup: critical UI first, light DOM next, heavy plugins last. */
 const PLUGIN_START_PHASES: readonly string[][] = [
+  // 404 page boot is a tiny standalone path; start it first so the kitten
+  // card renders before any SPA work. A no-op on normal SEQTA pages.
+  ["error-page-kitten"],
   ["themes", "animated-background"],
   [
     "timetable",
diff --git a/src/plugins/index.ts b/src/plugins/index.ts
index 039be718..9be02278 100644
--- a/src/plugins/index.ts
+++ b/src/plugins/index.ts
@@ -16,6 +16,7 @@ import messageFoldersPluginLazy from "./built-in/messageFolders/lazy";
 import enhancedNavigationPluginLazy from "./built-in/enhancedNavigation/lazy";
 import globalSearchPluginLazy from "./built-in/globalSearch/lazy";
 import gradeAnalyticsPluginLazy from "./built-in/gradeAnalytics/lazy";
+import errorPageKittenPluginLazy from "./built-in/errorPageKitten/lazy";
 
 // Initialize plugin manager
 const pluginManager = PluginManager.getInstance();
@@ -36,6 +37,7 @@ pluginManager.registerPlugin(messageFoldersPluginLazy);
 pluginManager.registerPlugin(enhancedNavigationPluginLazy);
 pluginManager.registerPlugin(globalSearchPluginLazy);
 pluginManager.registerPlugin(gradeAnalyticsPluginLazy);
+pluginManager.registerPlugin(errorPageKittenPluginLazy);
 
 export async function initializePlugins(): Promise<void> {
   await pluginManager.startAllPlugins();
diff --git a/src/plugins/syncablePluginDefaults.ts b/src/plugins/syncablePluginDefaults.ts
index e438e1fa..cb7a58dd 100644
--- a/src/plugins/syncablePluginDefaults.ts
+++ b/src/plugins/syncablePluginDefaults.ts
@@ -37,4 +37,5 @@ export const SYNCABLE_PLUGIN_SETTING_DEFAULTS: Record<
     passiveIndexing: true,
   },
   "grade-analytics": { cacheTtlHours: 24 },
+  "error-page-kitten": {},
 };
diff --git a/src/resources/error-page/kitten.png b/src/resources/error-page/kitten.png
new file mode 100644
index 00000000..1a94d5d0
Binary files /dev/null and b/src/resources/error-page/kitten.png differ
diff --git a/src/test/mocks/assetStub.ts b/src/test/mocks/assetStub.ts
new file mode 100644
index 00000000..50e1fdeb
--- /dev/null
+++ b/src/test/mocks/assetStub.ts
@@ -0,0 +1,2 @@
+// Jest stub for Vite asset imports (png, etc.): Vite returns a URL string.
+export default "/resources/error-page/kitten.png";
diff --git a/src/test/mocks/inlineStub.ts b/src/test/mocks/inlineStub.ts
new file mode 100644
index 00000000..23332858
--- /dev/null
+++ b/src/test/mocks/inlineStub.ts
@@ -0,0 +1,2 @@
+// Jest stub for Vite's `?inline` css imports: returns a CSS string.
+export default "/* test inline css */";
diff --git a/src/test/mocks/webextension-polyfill.ts b/src/test/mocks/webextension-polyfill.ts
index 21d57028..3bffe4a3 100644
--- a/src/test/mocks/webextension-polyfill.ts
+++ b/src/test/mocks/webextension-polyfill.ts
@@ -32,6 +32,9 @@ export default {
   storage: { local, onChanged },
   runtime: {
     sendMessage: jest.fn(async () => undefined),
+    getURL: jest.fn(
+      (path: string) => `chrome-extension://test/${String(path).replace(/^\/+/, "")}`,
+    ),
   },
 };