diff --git a/cloudbuild.yml b/cloudbuild.yml
index e12ce055881..98ec8462ed8 100644
--- a/cloudbuild.yml
+++ b/cloudbuild.yml
@@ -11,6 +11,10 @@ steps:
   id: 'diff'
   args: ['diff']
   waitFor: ['yarn']
+  env:
+  - 'COMMIT_SHA=$COMMIT_SHA'
+  - 'BRANCH_NAME=$BRANCH_NAME'
+  - 'REPO_NAME=$REPO_NAME'
 
 # Core.
 - name: 'gcr.io/cloud-builders/gcloud'
diff --git a/scripts/diff.js b/scripts/diff.js
index fde9e6f6897..77c57b41142 100755
--- a/scripts/diff.js
+++ b/scripts/diff.js
@@ -15,6 +15,7 @@
 // =============================================================================
 
 const {exec} = require('./test-util');
+const shell = require('shelljs');
 const {readdirSync, statSync, writeFileSync} = require('fs');
 const {join} = require('path');
 const fs = require('fs');
@@ -30,9 +31,45 @@ const dirs = readdirSync('.').filter(f => {
   return f !== 'node_modules' && f !== '.git' && statSync(f).isDirectory();
 });
 
-exec(
-    `git clone --depth=1 --single-branch ` +
-    `https://github.com/tensorflow/tfjs ${CLONE_PATH}`);
+let commitSha = process.env['COMMIT_SHA'];
+let branchName = process.env['BRANCH_NAME'];
+// If commit sha or branch name are null we are running this locally and are in
+// a git repository.
+if (commitSha == null) {
+  commitSha = exec(`git rev-parse HEAD`).stdout.trim();
+}
+if (branchName == null) {
+  branchName = exec(`git rev-parse --abbrev-ref HEAD`).stdout.trim();
+}
+console.log('commitSha: ', commitSha);
+console.log('branchName: ', branchName);
+
+// We cannot do --depth=1 here because we need to check out an old merge base.
+// We cannot do --single-branch here because we need multiple branches.
+exec(`git clone https://github.com/tensorflow/tfjs ${CLONE_PATH}`);
+
+console.log();  // Break up the console for readability.
+
+shell.cd(CLONE_PATH);
+
+// If we cannot check out the commit then this PR is coming from a fork.
+const res = shell.exec(`git checkout ${commitSha}`, {silent: true});
+const isPullRequestFromFork = res.code !== 0;
+
+// Only checkout the merge base if the pull requests comes from a
+// tensorflow/tfjs branch. Otherwise clone master and diff against master.
+if (!isPullRequestFromFork) {
+  console.log('PR is coming from tensorflow/tfjs. Finding the merge base...');
+  exec(`git checkout ${branchName}`);
+  const mergeBase = exec(`git merge-base master ${branchName}`).stdout.trim();
+  exec(`git fetch origin ${mergeBase}`);
+  exec(`git checkout ${mergeBase}`);
+  console.log('mergeBase: ', mergeBase);
+} else {
+  console.log('PR is coming from a fork. Diffing against master.');
+}
+shell.cd('..');
+console.log();  // Break up the console for readability.
 
 let triggerAllBuilds = false;
 let whitelistDiffOutput = [];
@@ -45,11 +82,11 @@ filesWhitelistToTriggerBuild.forEach(fileToTriggerBuild => {
   }
 });
 
-// Break up the console for readability.
-console.log();
+console.log();  // Break up the console for readability.
 
 let triggeredBuilds = [];
 dirs.forEach(dir => {
+  shell.rm('-f', `${dir}/diff`);
   const diffOutput = diff(`${dir}/`);
   if (diffOutput !== '') {
     console.log(`${dir} has modified files.`);
@@ -65,16 +102,17 @@ dirs.forEach(dir => {
   }
 });
 
-// Break up the console for readability.
-console.log();
+console.log();  // Break up the console for readability.
 
-// Filter the triggered builds to log by whether a cloudbuild.yml file exists
-// for that directory.
+// Filter the triggered builds to log by whether a cloudbuild.yml file
+// exists for that directory.
 triggeredBuilds = triggeredBuilds.filter(
     triggeredBuild => fs.existsSync(triggeredBuild + '/cloudbuild.yml'));
 console.log('Triggering builds for ', triggeredBuilds.join(', '));
 
 function diff(fileOrDirName) {
-  const diffCmd = `diff -rq ${CLONE_PATH}/${fileOrDirName} ./${fileOrDirName}`;
+  const diffCmd = `diff -rq ` +
+      `${CLONE_PATH}/${fileOrDirName} ` +
+      `${fileOrDirName}`;
   return exec(diffCmd, {silent: true}, true).stdout.trim();
 }