Doctor Who Season 4 Episode o Time Crash

Doctor Who Time Crash 0/13

After saying farewell to Martha, the Doctor encounters a problem with the TARDIS. Because of this the Fifth Doctor appears in the console room. The Tenth Doctor is gleeful at the meeting, but the Fifth Doctor is initially baffled, assuming his future incarnation is a deranged fan, possibly from LINDA.

The Tenth Doctor explains that he forgot to put the shields up after rebuilding the TARDIS and it collided with the Fifth Doctor's TARDIS (its earlier self) in the timestream. This is generating a paradox at the heart of the ship, which is powerful enough to rip a hole in the universe the (exact) size of Belgium. The Cloister Bell signals the impending end.

However, without a thought, the Tenth Doctor manipulates the TARDIS controls to manipulate a supernova into exact counterbalance; it cancels out the black hole caused by the paradox, so that all matter remains constant. This amazes the Fifth Doctor, but he quickly realises that the Tenth Doctor 'came up with' the solution only because he remembered this encounter. The Fifth Doctor says his farewells, and the Tenth Doctor tells the Fifth of the personality traits that he retained from his fifth self (trainers, the altering pitch of his voice and his spectacles), also telling him he loved being him and that he was "his" Doctor.

As he departs, the Fifth Doctor reminds the Tenth to raise his shields again, but it appears that he was too late, since the hull of the Titanic crashes through one of the TARDIS walls, as originally seen at the end of the last series.