diff --git a/src/xhrInterceptor/responseModifiers/index.js b/src/xhrInterceptor/responseModifiers/index.js
new file mode 100644
index 0000000..6a4573a
--- /dev/null
+++ b/src/xhrInterceptor/responseModifiers/index.js
@@ -0,0 +1,75 @@
+import MWOptionsWatcherClient from '../../common/mainWorldOptionsWatcher/Client.js';
+import {convertJSONToResponse, getResponseJSON} from '../utils.js';
+
+import demo from './demo.js';
+
+export const responseModifiers = [
+  demo,
+];
+
+// Content script target
+export const kCSTarget = 'TWPT-XHRInterceptorOptionsWatcher-CS';
+// Main world (AKA regular web page) target
+export const kMWTarget = 'TWPT-XHRInterceptorOptionsWatcher-MW';
+
+export default class ResponseModifier {
+  constructor() {
+    this.optionsWatcher = new MWOptionsWatcherClient(
+        Array.from(this.watchingFeatures()), kCSTarget, kMWTarget);
+  }
+
+  watchingFeatures(modifiers) {
+    if (!modifiers) modifiers = responseModifiers;
+
+    const union = new Set();
+    for (const m of modifiers) {
+      if (!m.featureGated) continue;
+      for (const feature of m.features) union.add(feature);
+    }
+    return union;
+  }
+
+  async #getMatchingModifiers(request) {
+    // First filter modifiers which match the request URL regex.
+    const urlModifiers = responseModifiers.filter(
+        modifier => request.$TWPTRequestURL.match(modifier.urlRegex));
+
+    // Now filter modifiers which require a certain feature to be enabled
+    // (feature-gated modifiers).
+    const featuresAreEnabled = await this.optionsWatcher.areEnabled(
+        Array.from(this.watchingFeatures(urlModifiers)));
+
+    // #!if !production
+    if (Object.keys(featuresAreEnabled).length > 0) {
+      console.info(
+          '[XHR Interceptor - Response Modifier] Requested features',
+          featuresAreEnabled, 'for request', request.$TWPTRequestURL);
+    }
+    // #!endif
+
+    return urlModifiers.filter(modifier => {
+      return !modifier.featureGated || modifier.isEnabled(featuresAreEnabled);
+    });
+  }
+
+  async intercept(request, response) {
+    const matchingModifiers = await this.#getMatchingModifiers(request);
+
+    // If we didn't find any matching modifiers, return the response right away.
+    if (matchingModifiers.length === 0) return response;
+
+    // Otherwise, apply the modifiers sequentially and set the new response.
+    let json = getResponseJSON({
+      responseType: request.xhr.responseType,
+      response: request.xhr.response,
+      $TWPTRequestURL: request.$TWPTRequestURL,
+      $isArrayProto: request.$isArrayProto,
+    });
+    for (const modifier of matchingModifiers) {
+      json = await modifier.interceptor(request, json);
+    }
+    response = convertJSONToResponse(request, json);
+    request.$newResponse = response;
+    request.$responseModified = true;
+  }
+}