diff --git a/doc/Makefile b/doc/Makefile
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -22,7 +22,7 @@ hg.1.txt: hg.1.gendoc.txt
 	touch hg.1.txt
 
 hg.1.gendoc.txt: $(GENDOC)
-	${PYTHON} gendoc.py > $@.tmp
+	${PYTHON} gendoc.py hg.1.gendoc > $@.tmp
 	mv $@.tmp $@
 
 hgrc.5: ../mercurial/help/config.txt
diff --git a/doc/gendoc.py b/doc/gendoc.py
--- a/doc/gendoc.py
+++ b/doc/gendoc.py
@@ -1,3 +1,8 @@
+"""usage: %s DOC ...
+
+where DOC is the name of a document
+"""
+
 import os, sys, textwrap
 # import from the live mercurial repo
 sys.path.insert(0, "..")
@@ -168,4 +173,11 @@ def allextensionnames():
     return extensions.enabled().keys() + extensions.disabled().keys()
 
 if __name__ == "__main__":
-    showdoc(sys.stdout)
+    doc = 'hg.1.gendoc'
+    if len(sys.argv) > 1:
+        doc = sys.argv[1]
+
+    if doc == 'hg.1.gendoc':
+        showdoc(sys.stdout)
+    else:
+        showtopic(sys.stdout, sys.argv[1])