Synopsis: The Seventh Doctor stumbles across some old junk in the TARDIS that reminds him of an incident in the past. He tells Ace about a time when his Fourth incarnation materialized inside a sunken submarine. From a ship on the ocean's surface comes a friendly voice. The man from above identifies himself as U.N.I.T.'s scientific advisor. He claims to be tracking down the Silurians who had sunk the submarine. The Fourth Doctor is shocked to discover that the "U.N.I.T. scientific advisor" is none other than his Third self.
Thus connected to his past self only by a short wave radio, the Fourth Doctor must find a way to assist the Third to defeat the Silurians, without actually coming into contact with his old self. The fate of the Earth—and his own timeline—depends on it!
Notes: This story is set, from the Third Doctor's perspective, in the midst of the televised adventure, The Sea Devils.