Martha is leaving the TARDIS to deal with her family and the Doctor is left alone. He activates the TARDIS, which goes crazy, and he starts to phase about. The Doctor goes to work on the TARDIS console, only to find someone else working on it: his fifth incarnation.
The 10th Doctor immediately recognises his past self but the 5th Doctor has no idea who the skinny stranger in his TARDIS is. The 5th Doctor determines that there’s a temporal alert as if two TARDISs have merged, while the 10th Doctor seems unconcerned. The 5th Doctor realises that the energy overload will blow a hole in the space-time continuum, then realises who the 10th Doctor is: a fan who somehow got into the TARDIS.
The Cloister Bell rings and the 10th Doctor starts activating the console and venting the thermal buffers, admitting he lowered the TARDIS shields by accident. He manages to generate a supernova and a black hole, canceling the two out. The 5th Doctor is impressed and the 10th Doctor admits that he remembered what to do, because he remembered himself as the 5th Doctor seeing himself doing it. The 5th Doctor finally accepts his future self as himself as the timestreams start to split apart again. The 10th Doctor holds him there long enough to admit he loved being the 5th Doctor.
The 5th Doctor fades away but not before mentioning his future self should put the shields up. Then a ship’s prow breaks through the side of the TARDIS...