diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1d93609..c568e4b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -32,6 +32,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/) and this p
 
 ### Fixed
 
+- Fixes [#1218](https://github.com/eamodio/vscode-gitlens/issues/1218) - Opening Ahead/Behind files isn't showing the desire diff (e.g. diff with the merge base)
 - Fixes [#1255](https://github.com/eamodio/vscode-gitlens/issues/1255) - Repository folders are missing repository actions (e.g. favorites, close repo, etc)
 - Fixes [#1246](https://github.com/eamodio/vscode-gitlens/issues/1246) - Gutter Blame avatar does not use Gravatar fallback style
 - Fixes [#1208](https://github.com/eamodio/vscode-gitlens/issues/1208) - Connect to Github notification is noisy
diff --git a/src/views/nodes/compareBranchNode.ts b/src/views/nodes/compareBranchNode.ts
index c556ae0..ef1c318 100644
--- a/src/views/nodes/compareBranchNode.ts
+++ b/src/views/nodes/compareBranchNode.ts
@@ -69,6 +69,9 @@ export class CompareBranchNode extends ViewNode<BranchesView | CommitsView | Rep
 			const aheadBehindCounts = await Container.git.getAheadBehindCommitCount(this.branch.repoPath, [
 				GitRevision.createRange(behind.ref1, behind.ref2, '...'),
 			]);
+			const mergeBase = await Container.git.getMergeBase(this.branch.repoPath, behind.ref1, behind.ref2, {
+				forkPoint: true,
+			});
 
 			this._children = [
 				new ResultsCommitsNode(
@@ -81,7 +84,7 @@ export class CompareBranchNode extends ViewNode<BranchesView | CommitsView | Rep
 						comparison: behind,
 						direction: 'behind',
 						files: {
-							ref1: this.compareWithWorkingTree ? '' : behind.ref1,
+							ref1: this.compareWithWorkingTree ? '' : mergeBase ?? behind.ref1,
 							ref2: behind.ref2,
 							query: this.getBehindFilesQuery.bind(this),
 						},
@@ -104,7 +107,7 @@ export class CompareBranchNode extends ViewNode<BranchesView | CommitsView | Rep
 						comparison: ahead,
 						direction: 'ahead',
 						files: {
-							ref1: ahead.ref1,
+							ref1: mergeBase ?? ahead.ref1,
 							ref2: this.compareWithWorkingTree ? '' : ahead.ref2,
 							query: this.getAheadFilesQuery.bind(this),
 						},
diff --git a/src/views/nodes/compareResultsNode.ts b/src/views/nodes/compareResultsNode.ts
index 4c387d8..361e4b1 100644
--- a/src/views/nodes/compareResultsNode.ts
+++ b/src/views/nodes/compareResultsNode.ts
@@ -77,6 +77,9 @@ export class CompareResultsNode extends ViewNode<SearchAndCompareView> {
 			const aheadBehindCounts = await Container.git.getAheadBehindCommitCount(this.repoPath, [
 				GitRevision.createRange(behind.ref1 || 'HEAD', behind.ref2, '...'),
 			]);
+			const mergeBase = await Container.git.getMergeBase(this.repoPath, behind.ref1, behind.ref2, {
+				forkPoint: true,
+			});
 
 			this._children = [
 				new ResultsCommitsNode(
@@ -89,7 +92,7 @@ export class CompareResultsNode extends ViewNode<SearchAndCompareView> {
 						comparison: behind,
 						direction: 'behind',
 						files: {
-							ref1: behind.ref1,
+							ref1: behind.ref1 === '' ? '' : mergeBase ?? behind.ref1,
 							ref2: behind.ref2,
 							query: this.getBehindFilesQuery.bind(this),
 						},
@@ -110,7 +113,7 @@ export class CompareResultsNode extends ViewNode<SearchAndCompareView> {
 						comparison: ahead,
 						direction: 'ahead',
 						files: {
-							ref1: ahead.ref1,
+							ref1: mergeBase ?? ahead.ref1,
 							ref2: ahead.ref2,
 							query: this.getAheadFilesQuery.bind(this),
 						},