In present-day London, the Doctor and Liz are working on the TARDIS console when Sergeant Mike Yates arrives in the laboratory with a package from UNIT's Australian bureau, a fragment of an alien spaceship found in the belly of a shark. The Doctor tunes the TARDIS’ Space-Time Visualiser to the frequency of the omicron radiation trace on the fragment, and creates what appears to be a holographic image of the fragment's path through history, terminating at the moment of its creation. As Yates leaves to report to the Brigadier, however, Liz realises that the “image” is in fact a time bridge, through which solid material can pass; the Doctor is in fact making another attempt to escape from his exile. The Doctor passes through the bridge to explore the island on the other side, but the Visualiser's power accumulators begin to overload. While trying to warn him, Liz accidentally falls through the time bridge and is stranded on the other side with him when the accumulators short out.