The Fifth Doctor demands to know the identity of his new companion whilst the Tenth marvels at what has happened, seeming to shrug off the fact the result of their meeting could destroy the universe. Again his erstwhile self demands to know who he is, but the Tenth Doctor ignores him and examines his previous incarnation further. He notes that he appears somewhat older than usual, presumably because of the time differential of having the two of them in the same place at the same time; normality will be resumed when he is returned to his own TARDIS.
He then stands back and admires the Fifth Doctor’s costume, complimenting him on trying to encompass a stick of celery into the ensemble. He is silenced by his former incarnation, who removes his hat and turns to examine the console. The Tenth Doctor takes the opportunity to examine the back of his own head, but asks that his past self put his hat back on when he notices a bald spot.
The Fifth Doctor turns to face him and moans that he has changed the theme of the TARDIS interior. He then returns to his work and, much to the amusement of the Tenth Doctor, puts on his glasses:
“You don’t even need them, you just think they make you look a bit clever”
Another alert sounds and the Fifth Doctor notes that it means two TARDISes have collided, merging together. Two time zones are at war at the heart of the TARDIS, creating a paradox that could punch a whole in the universe the size of Belgium. The Tenth Doctor offers him the Sonic Screwdriver to try and make repairs, but he declines the offer. Eventually he turns to his companion and a look of realisation slides across his face.
“You’re a fan!”
He resumes his work leaving his counterpart somewhat dumbfounded by his ignorance. He explains that during his travels he has been noticed and as a result people start fan-groups in favour of his good work. The Tenth Doctor tells him bluntly who he is, a future incarnation.
“Check out this bone structure Doctor because one day you’ll be shaving it”
The Cloister Bell sounds and the Fifth Doctor notes that the collision that has occurred could create a black hole capable of destroying the universe. The Tenth Doctor explains it is his fault, he forgot to activate the defence shields of his TARDIS when he was rebuilding it. He hurriedly jabs at the controls of the ship and the Fifth Doctor notes he is about to blow up the ship. The Tenth Doctor insists:
“Only way out”
“Who told you that?”
“You told me that!”
There is a flash of light but moments later the two Doctors are still alive inside the undamaged ship. The supernova created by the TARDIS has cancelled out the black hole caused by the collision thereby maintaining stability. The Tenth Doctor explains that he only knew what to do because he remembered being the Fifth Doctor and seeing his future self activate the controls.
“You only knew what to do because I saw you do it”
The TARDISes begin to separate and the Tenth Doctor begins to say goodbye, asking whereabouts in his existence the Fifth Doctor has reached. He tells him that the Master has reappeared recently and his other self asks if he still has “that rubbish beard”. The Tenth Doctor tells him he doesn’t, but he did have a wife.
The Fifth Doctor begins to fade away and they thank each other. The Tenth Doctor suddenly remembers something and flicks a switch on the console, causing his past incarnation to return. He hands him his hat, which he has left behind and then says his final farewell:
“You know, I loved being you. Back when I first started at the very beginning I was always trying to be old and grumpy and important, like you do when you’re young…and then I was you. And it was all dashing about and playing cricket and my voice going all squeaky when I shouted, I still do that, the voice thing, I got that from you. Oh, and the trainers…because you know what Doctor, you were my Doctor”
The Fifth Doctor fades from view with one final farewell and the Tenth resumes his place at the console. A voice then rings out around him:
“Oh Doctor, remember to put your shields up!”
The Doctor flicks a switch and suddenly there is the sound of an alarm, the wall before him is blown away and he is thrown to the floor. He looks up to find the front of an ocean liner embedded in the side of the TARDIS, and scrambles to reach a buoyancy aid that has fallen from the vessel. He turns it over to reveal the name of the ship: the Titanic.