Careering out of control through time and space, the TARDIS materialises aboard a massive spaceship which has emerged from a wormhole at the very edge of the universe. The Doctor and Donna leave the time machine so that it can repair itself, only to watch in shock as it vanishes. The TARDIS has been spooked by a hostile presence that it has detected aboard the vessel, but all the travellers can find is an ancient, virtually inert robot and a strange knocking on the hull. Yet while the ship's computer confirms the absence of other life signs, the Doctor and Donna are soon confronted by their own distorted doppelgangers.