You've likely at least heard of Season 6B - fitting between the conclusion of The War Games and the beginning of Spearhead From Space - as a layman's term. The gap in continuity between the last stand of the Second Doctor and his new beginning post-regeneration as an exile to Earth was to an extent created by Terrance Dicks, who subsequently revisited it in Players by way of a flashback sequence for the Sixth Doctor in which he literally relives his past as the little chap with an initial fondness for new & interesting headgear and a recorder never far away.
Something similar occurs in The Two Doctors, an innocent Gumblejack fishing trip suffering a setback as twinges of that very past come back to haunt the colourfully-coated man he is nowadays........so begins a sub-season of sorts, as they cross paths at convenient temporal intersections!
PERI: Doctor, er, are you all right?But of course, according to Dicks' later World Game, he was offered a deal by the Celestial Intervention Agency- become an agent of theirs and earn at least a temporary reprieve from his sentence. Having accepted, he was spirited off to Waterloo to stop the Players. And his success in that first mission earns him a nice new TARDIS interior and a reunion with Jamie, who's had his mind altered to believe that Victoria is away studying graphology- though she left long ago.
DOCTOR: Of course I'm not all right! What happened?
PERI: I think you fainted.
DOCTOR: I never faint. I remember now. I felt a weakness. I felt a weakness and then I, I was in another place.
PERI: Can I get you anything? Celery! That's what you need.
DOCTOR: Celery, yes. And the tensile strength of jelly babies! But I, I had a clarinet. Or was it a flute? Something you blew into.
PERI: A glass of water?
DOCTOR: Water? No, don't think so. A recorder! That's what it was. Some kind of mind lock.
PERI: Doctor, you're not making any sense.
DOCTOR: I am making perfect sense. I was being put to death!
Having been given the assignment to look into the activities of Space Station Chimera, the younger Doctor and his best friend from the Highlands are back! And time has not been kind to that mop-top, as its gone from the black we're so used to seeing to grey. The change is explained away by Dicks in World Game as a result of stress brought on by the death of his Time Lady companion Serena, shot through both hearts. No time to mourn, though.......
JAMIE: It's like twenty castles in the sky! Are you sure we've come to the right place?The object of his ire is a teleport control, allowing the Time Lords to remotely control the TARDIS. ''You'd think I'd never flown a TARDIS solo!''. A chat with old friend Dastari does little to improve his mood, though it does explain why he's here in the first place.
DOCTOR: Of course I am.
JAMIE: We don't usually get to where you say we're going.
DOCTOR : I got Victoria to where she wanted to go, though why she wants to learn graphology, I've no idea.
JAMIE: Ah. Will we ever get back to her, though?
DOCTOR : Of course we will.
JAMIE: I'll believe that when I see it.
DOCTOR : At the moment, we have other things to worry about. Look at this!
DOCTOR : Don't chide me, Dastari. I'm simply a messenger. Officially, I'm here quite unofficially.Having failed to win him over with his diplomatic skills and got himself captured, the Doctor needs help- which arrives in the form of himself. And that one quickly finds his old companion , which probably rather baffles his latest!
DASTARI: You'll explain that paradox, I know.
DOCTOR : I'm a pariah, exiled from Time Lord society, so they can always deny sending me.
DASTARI: And why have they sent you?
DOCTOR : They have been monitoring the experiments in time travel of the Professors Kartz and Reimer. They want them stopped.
DASTARI: I see. And how do the Time Lords equate that with a policy of complete neutrality?
DOCTOR : They don't have to. As I said, I have no official existence, so they can always deny sending me.
DASTARI: Oh, typical. Typical hypocrisy.
DOCTOR: Jamie?Deducing that he can't possibly be here if his past self has been killed, time to go on a rescue mission.
DOCTOR: It's Jamie. How did he get here? He should be with me.
PERI: Well, he's not now, Doctor. Not any more.
DOCTOR: No, that's right. But if he's here, where am I? I must have been here, Peri.
PERI: You mean at some past time?
DOCTOR: It's all right, Jamie.
JAMIE: Keep away from me!
JAMIE: So you don't think the Doctor's dead? I mean, my Doctor.Which leads to their first face to face meeting, though they have shared billing for part of Players as the Sixth remembers being the Second for a post-War Games meeting with Lady Jennifer Buckingham, Lieutenant Carstairs & Winston Churchill.
DOCTOR: No, I don't, Jamie. And if I'm not dead in that form, it means my theory about the embolism is also wrong. Well, this begins to have all the hallmarks of a conspiracy.
PERI: What sort of conspiracy?
JAMIE: A plot.
DOCTOR: That's right, Jamie. A plot to kidnap me and Dastari as well. And he's about the only bio-geneticist in the galaxy capable of isolating the symbiotic nuclei of a Time Lord.
PERI: So that's how you control the TARDIS. Symbiosis.
DOCTOR: If the Sontarans ever get unlimited access to time travel, they'll be invincible. We must find out where they're holding me.
JAMIE: How could we do that? They could be anywhere.
DOCTOR: I made contact with myself before during that mind-slip. I'll try telepathy. It's about our only chance.
DOCTORS: Snap!Something of a dream come true for Colin Baker, a noted fan of his illustrious Doctoral forebear..................
DOCTOR: I've come a long way for you!
DOCTOR : Naturally. Don't expect any thanks.