 When a gas accident causes McCoy to go temporarily insane, he escapes to strange planet. There the search party discovers a device left by a superior, vanished civilization, a time portal which plays the history of earth for them- but then Bones jumps through it into the past, causing a change in history important enough to make the Enterprise vanish. Kirk and Spock, who fortunately made a tricorder recording, must attempt to go through to just before McCoys arrival and stop him changing history in the US during the Great Depression, where they have no advanced technology available.