diff --git a/hgext3rd/undo.py b/hgext3rd/undo.py
--- a/hgext3rd/undo.py
+++ b/hgext3rd/undo.py
@@ -11,13 +11,17 @@
 from mercurial.i18n import _
 
 from mercurial import (
+    cmdutil,
+    commands,
     dispatch,
     error,
     extensions,
+    obsolete,
     registrar,
     revlog,
     revset,
     revsetlang,
+    scmutil,
     smartset,
     transaction,
     util,
@@ -29,6 +33,10 @@
     nullid,
 )
 
+from hgext3rd import (
+    inhibit,
+)
+
 cmdtable = {}
 command = registrar.command(cmdtable)
 
@@ -276,6 +284,80 @@
     revs = _getolddrafts(repo, reverseindex)
     return smartset.baseset(revs)
 
+# Undo:
+
+@command('undo', [
+    ('n', 'index', 1, _("how many steps to undo back")),
+])
+def undo(ui, repo, *args, **opts):
+    """ perform an undo
+
+    Undoes an undoable command.  An undoable command is one that changed atleast
+    one of the following three: bookmarks, working copy parent or changesets.
+    Note that this specifically does not include commands like log.  It will
+    include update if update changes the working copy parent (you update to a
+    changeset that isn't the current one).  Note that commands that edit public
+    repos can't be undone (specifically push).
+
+    Undo does not preserve the working copy changes.
+    """
+    reverseindex = opts.get("index")
+
+    if repo is not None:
+        with repo.wlock(), repo.lock(), repo.transaction("undo"):
+            cmdutil.checkunfinished(repo)
+            cmdutil.bailifchanged(repo)
+            repo = repo.unfiltered()
+            _undoto(ui, repo, reverseindex)
+            # store undo data
+def _undoto(ui, repo, reverseindex):
+    # undo to specific reverseindex
+    try:
+        nodedict = _readindex(repo, reverseindex)
+    except IndexError:
+        raise error.Abort(_("index out of bounds"))
+
+    # bookmarks
+    bookstring = _readnode(repo, "bookmarks.i", nodedict["bookmarks"])
+    booklist = bookstring.split("\n")
+    # copy implementation for bookmarks
+    itercopy = []
+    for mark in repo._bookmarks.iteritems():
+        itercopy.append(mark)
+    for mark in itercopy:
+        commands.bookmark(ui, repo, mark[0], delete=True)
+    for mark in booklist:
+        if mark is not None and mark != "":
+            kv = mark.rsplit(" ", 1)
+            commands.bookmark(ui, repo, kv[0], rev=kv[1])
+
+    # working copy parent
+    workingcopyparent = _readnode(repo, "workingparent.i",
+                                  nodedict["workingparent"])
+    revealcommits(ui, repo, workingcopyparent)
+    commands.update(ui, repo, rev=workingcopyparent)
+
+    # visible changesets
+    addedrevs = revsetlang.formatspec('olddraft(0) - olddraft(%d)',
+                                      reverseindex)
+    revs = repo.revs(addedrevs)
+    with repo.lock():
+        for r in revs:
+            hidecommits(repo, repo.changectx(r))
+    removedrevs = revsetlang.formatspec('olddraft(%d) - olddraft(0)',
+                                        reverseindex)
+    revealcommits(ui, repo, removedrevs)
+
+# hide and reveal commits
+
+def hidecommits(repo, commit):
+    obsolete.createmarkers(repo, [[commit,[]]])
+
+def revealcommits(ui, repo, rev):
+    revr = scmutil.revrange(repo, [rev])
+    contexts = (repo.changectx(repo[r].node()) for r in revr)
+    inhibit.revive(contexts)
+
 # Tools
 
 def _invertindex(rlog, indexorreverseindex):