diff --git a/extract-docker.sh b/extract-docker.sh
index 5f4e9b8..53c4927 100755
--- a/extract-docker.sh
+++ b/extract-docker.sh
@@ -2,6 +2,10 @@
 
 if [ $# -lt 2 ]; then
     echo $0 image dir
+    echo
+    echo This extracts \'dir\' from the docker image named \'image\' into the
+    echo current directory. NOTE: This removes the existing directory named
+    echo \'dir\' first.
     exit 1
 fi