diff --git a/test-merge.sh b/test-merge.sh
new file mode 100755
index 0000000..9d31653
--- /dev/null
+++ b/test-merge.sh
@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+set -x
+
+# The tomono script is tangled right next to the test script
+export PATH="$PWD:$PATH"
+
+d="$(mktemp -d)"
+cd "$d"
+
+# Two reasonably complex repos with lots of changes but not too many. And both
+# with a ‘master’ branch.
+>remotes.txt cat <<EOF
+https://github.com/git/git.git git
+https://git.code.sf.net/p/sbcl/sbcl sbcl
+EOF
+
+cat remotes.txt | while read url dir ; do
+	git clone "$url" "$dir"
+	(
+		cd "$dir"
+		git log --before 1.year.ago -1 --format=%h | xargs git reset --hard
+	)
+	echo "$PWD/$dir $dir" >> locals.txt
+done
+
+<locals.txt tomono
+
+# Now we have a repo core with SBCL and Git, both where they were 1y ago. Update
+# those and merge in the changes.
+
+cat remotes.txt | while read _ dir ; do
+	(
+		cd "$dir"
+		git reset --hard HEAD@{1}
+	)
+done
+
+cd core
+git fetch --all
+git checkout master
+
+cat ../remotes.txt | while read _ dir; do
+	git merge --no-edit -X subtree="$dir/" "$dir/master"
+done
+
+echo "Successfully merged in 1 year worth of changes from both remotes into $PWD"