In a moment I am sure will never be repeated, the Doctor, the Black Guardian, and Mawdryn all agree that the two different versions of the Brigadier should not meet because that would be very, very bad.
Granted, they don’t talk to each other about that, but they all independently come to that conclusion.
And man, do they try to prevent that from happening. The Doctor won’t give up his last eight regenerations to help Mawdryn and his seven buddies end their unlives of eternal suffering, so he’s taking the 1983 Brigadier back in the TARDIS.
Small problem there: the TARDIS won’t quite leave the ship without Tegan and Nyssa coming down with the same disease Mawdryn has. The Doctor tries to reverse the direction of the neutron flow, but that just causes the women to age in reverse.
Meanwhile, the Black Guardian tells Turlough to lock the 1977 Brigadier up. The Doctor also tells him to zap that Brigadier back to Earth in his time with the Transmat once the TARDIS gets there. And…that doesn’t work because the TARDIS never really leaves. Even the undead Mawdryn tries to lock the ’77 Brigadier up to prevent the massive energy explosion that would, well, kill a whole lot of people if not the entire universe.
Seeing no other options to help the women, the Doctor agrees to give up his regenerations (and Time Lord status) to give Mawdryn and friends death and the women a cure. Will that be the end of the road for the Doctor?
Nah! The two Brigadiers meet, and the explosion that results allows the Doctor to cure Nyssa and Tegan while killing Mawdryn and his friends. And the Doctor didn’t have to give up a thing. Plus, with both Brigadiers unconscious, it doesn’t take much effort to fix the time stream. ’83 Brigadier wakes up and Nyssa takes him deep into the TARDIS. The other is groggy, so the Doctor takes him back to ’77, and he wakes up once they leave him in the grass, no doubt causing his memory loss.
And, with all that done and both Brigadiers back where they started, there’s not much left to do but try and get back to Turlough before Mawdryn’s ship self-destructs. But Turlough was already on the TARDIS, and the Doctor opts to let him stay because, you know, letting one of the Black Guardian’s unwilling assassins on board the TARDIS must be a good idea.
As for the Brigadier, well, we haven’t seen the last of him either.