diff --git a/src/plugins/discourse/createGraph.test.js b/src/plugins/discourse/createGraph.test.js
index 8e6052d..00f8ef3 100644
--- a/src/plugins/discourse/createGraph.test.js
+++ b/src/plugins/discourse/createGraph.test.js
@@ -83,6 +83,12 @@ describe("plugins/discourse/createGraph", () => {
     findUsername() {
       throw new Error("Method findUsername should be unused by createGraph");
     }
+    topicById() {
+      throw new Error("Method topicById should be unused by createGraph");
+    }
+    postsInTopic() {
+      throw new Error("Method postsInTopic should be unused by createGraph");
+    }
   }
 
   function example() {
diff --git a/src/plugins/discourse/mirrorRepository.js b/src/plugins/discourse/mirrorRepository.js
index 63c1ad8..9dd3d2b 100644
--- a/src/plugins/discourse/mirrorRepository.js
+++ b/src/plugins/discourse/mirrorRepository.js
@@ -67,6 +67,19 @@ export interface ReadRepository {
    * Note: input username is case-insensitive.
    */
   findUsername(username: string): ?string;
+
+  /**
+   * Gets a Topic by ID.
+   */
+  topicById(id: TopicId): ?Topic;
+
+  /**
+   * Gets a number of Posts in a given Topic. Starting with the first post,
+   * ordered by `indexWithinTopic`.
+   *
+   * numberOfPosts: the maximum number of posts to get (may return fewer).
+   */
+  postsInTopic(topicId: TopicId, numberOfPosts: number): $ReadOnlyArray<Post>;
 }
 
 export type SyncHeads = {|
@@ -278,6 +291,36 @@ export class SqliteMirrorRepository
       }));
   }
 
+  topicById(id: TopicId): ?Topic {
+    const res = this._db
+      .prepare(
+        dedent`\
+        SELECT
+          id,
+          category_id,
+          title,
+          timestamp_ms,
+          bumped_ms,
+          author_username
+        FROM topics
+        WHERE id = :id`
+      )
+      .get({id});
+
+    if (!res) {
+      return null;
+    }
+
+    return {
+      id: res.id,
+      categoryId: res.category_id,
+      title: res.title,
+      timestampMs: res.timestamp_ms,
+      bumpedMs: res.bumped_ms,
+      authorUsername: res.author_username,
+    };
+  }
+
   posts(): $ReadOnlyArray<Post> {
     return this._db
       .prepare(
@@ -304,6 +347,35 @@ export class SqliteMirrorRepository
       }));
   }
 
+  postsInTopic(topicId: TopicId, numberOfPosts: number): $ReadOnlyArray<Post> {
+    return this._db
+      .prepare(
+        dedent`\
+        SELECT
+          id,
+          timestamp_ms,
+          author_username,
+          topic_id,
+          index_within_topic,
+          reply_to_post_index,
+          cooked
+        FROM posts
+        WHERE topic_id = :topic_id
+        ORDER BY index_within_topic ASC
+        LIMIT :max`
+      )
+      .all({topic_id: topicId, max: numberOfPosts})
+      .map((x) => ({
+        id: x.id,
+        timestampMs: x.timestamp_ms,
+        authorUsername: x.author_username,
+        topicId: x.topic_id,
+        indexWithinTopic: x.index_within_topic,
+        replyToPostIndex: x.reply_to_post_index,
+        cooked: x.cooked,
+      }));
+  }
+
   users(): $ReadOnlyArray<string> {
     return this._db
       .prepare("SELECT username FROM users")
diff --git a/src/plugins/discourse/mirrorRepository.test.js b/src/plugins/discourse/mirrorRepository.test.js
index 89a4d52..99af8a0 100644
--- a/src/plugins/discourse/mirrorRepository.test.js
+++ b/src/plugins/discourse/mirrorRepository.test.js
@@ -302,4 +302,96 @@ describe("plugins/discourse/mirrorRepository", () => {
       if (error) throw error;
     }).toThrow("FOREIGN KEY constraint failed");
   });
+
+  it("topicById gets the matching topics", () => {
+    // Given
+    const db = new Database(":memory:");
+    const url = "http://example.com";
+    const repository = new SqliteMirrorRepository(db, url);
+    const topic1: Topic = {
+      id: 123,
+      categoryId: 42,
+      title: "Sample topic 1",
+      timestampMs: 456789,
+      bumpedMs: 456999,
+      authorUsername: "credbot",
+    };
+    const topic2: Topic = {
+      id: 456,
+      categoryId: 42,
+      title: "Sample topic 2",
+      timestampMs: 456789,
+      bumpedMs: 456999,
+      authorUsername: "credbot",
+    };
+
+    // When
+    repository.addTopic(topic1);
+    repository.addTopic(topic2);
+    const actualT1 = repository.topicById(topic1.id);
+    const actualT2 = repository.topicById(topic2.id);
+
+    // Then
+    expect(actualT1).toEqual(topic1);
+    expect(actualT2).toEqual(topic2);
+  });
+
+  it("postsInTopic gets the number of posts requested", () => {
+    // Given
+    const db = new Database(":memory:");
+    const url = "http://example.com";
+    const repository = new SqliteMirrorRepository(db, url);
+    const topic: Topic = {
+      id: 123,
+      categoryId: 1,
+      title: "Sample topic",
+      timestampMs: 456789,
+      bumpedMs: 456999,
+      authorUsername: "credbot",
+    };
+    const p1: Post = {
+      id: 100,
+      topicId: 123,
+      indexWithinTopic: 0,
+      replyToPostIndex: null,
+      timestampMs: 456789,
+      authorUsername: "credbot",
+      cooked: "<p>Valid post</p>",
+    };
+    const p2: Post = {
+      // Deliberately scramble id, order of `indexWithinTopic` should matter.
+      id: 121,
+      topicId: 123,
+      indexWithinTopic: 1,
+      replyToPostIndex: null,
+      timestampMs: 456888,
+      authorUsername: "credbot",
+      cooked: "<p>Follow up 1</p>",
+    };
+    const p3: Post = {
+      id: 102,
+      topicId: 123,
+      indexWithinTopic: 2,
+      replyToPostIndex: null,
+      timestampMs: 456999,
+      authorUsername: "credbot",
+      cooked: "<p>Follow up 2</p>",
+    };
+
+    // When
+    repository.addTopic(topic);
+    // Deliberately scramble the adding order, order of `indexWithinTopic` should matter.
+    repository.addPost(p3);
+    repository.addPost(p1);
+    repository.addPost(p2);
+    const posts0 = repository.postsInTopic(topic.id, 0);
+    const posts2 = repository.postsInTopic(topic.id, 2);
+    const posts5 = repository.postsInTopic(topic.id, 5);
+
+    // Then
+    // Note: these are in order, starting from the opening post.
+    expect(posts0).toEqual([]);
+    expect(posts2).toEqual([p1, p2]);
+    expect(posts5).toEqual([p1, p2, p3]);
+  });
 });
diff --git a/src/plugins/initiatives/discourse.test.js b/src/plugins/initiatives/discourse.test.js
index 0ea8a44..5313861 100644
--- a/src/plugins/initiatives/discourse.test.js
+++ b/src/plugins/initiatives/discourse.test.js
@@ -7,6 +7,7 @@ import {
   type DiscourseQueries,
 } from "./discourse";
 import {type Initiative} from "./initiative";
+import type {ReadRepository} from "../discourse/mirrorRepository";
 import type {Topic, Post, CategoryId, TopicId} from "../discourse/fetch";
 import {NodeAddress} from "../../core/graph";
 import dedent from "../../util/dedent";
@@ -156,6 +157,14 @@ describe("plugins/initiatives/discourse", () => {
     }
   });
 
+  describe("DiscourseQueries", () => {
+    it("should be a subset of the ReadRepository interface", () => {
+      const _unused_toDiscourseQueries = (
+        x: ReadRepository
+      ): DiscourseQueries => x;
+    });
+  });
+
   describe("DiscourseInitiativeRepository", () => {
     it("uses topicsInCategories to find initiative topics", () => {
       // Given