Episode 15 of "Lost"
s01e15 Homecoming

After the missing Claire returns with no recollection of what has happened since before the doomed flight of Oceanic 815, Jack and Locke formulate a plan of defense against her kidnapper, the mysterious Ethan (guest-star William Mapother), who threatens to kill off the other survivors one by one as the days go by unless Claire is returned to him. Meanwhile, the disappoint of Charlie feels of Claire not remembering him triggers recollections of a woman who had let down in the past.