In the first scene aboard the Valiant, the Master reminds the Doctor of their past encounters with the Axons and the Sea Devils. These alien races appeared in the 1971 serial The Claws of Axos and the 1972 serial The Sea Devils, both of which starred Jon Pertwee as the Doctor and Roger Delgado as the Master.

The Master: I remember the days when the Doctor, oh, that famous Doctor...was waging a Time War, battling Sea Devils and Axons, he sealed the Rift at the Medusa Cascade, single-handed. And look at him now, stealing screwdrivers, how did he ever come to this? Oh yeah! Me!
The Doctor: I just... need you to listen.
The Master: No, it's my turn. Revenge. Best served hot!

Professor Docherty: Oh god, I miss Countdown... Never been the same since Des took over... Both Deses... What's the plural for Des? Desie?...Desse?

Soldier: We're getting slaughtered!
Jack: Yeah, that happens to me a lot.

The Master: Dying in your arms. Happy now?
The Doctor: You’re not dying, don’t be stupid. It’s only a bullet. Just regenerate.
The Master: No.
The Doctor: One little bullet. Come on.
The Master: I guess you don’t know me so well. I refuse.
The Doctor: Regenerate! Just regenerate! Please. Please! Just regenerate! Come on!
The Master: And spend the rest of my life imprisoned with you?
The Doctor: You’ve got to! Come on. It can’t end like this. You and me, all the things we’ve done. Axons, remember the Axons? And the Daleks. We’re the only two left. Everyone else… regenerate!
The Master: How about that? I win. Will it stop, Doctor? The drumming? Will it stop?
(The Master dies)

Jack: But I keep wondering...what about ageing? 'Cause I can't die but I keep getting older...the odd little grey hair, you know? What happens if I live for a million years?
The Doctor: (amused) I really don't know.
Jack: Heh, okay vanity, sorry. Yeah, can't help it. Used to be a poster boy, when I was a kid, living in the Boeshane Peninsula. Tiny place. I was the first ever to be signed up for the Time Agency. They were so proud of me. The Face of Boe, they called me. (smiles) I'll see you.
(He runs off)
The Doctor: (quietly) No..
Martha: It can't be.
The Doctor: No...definitely not...no!
(Martha laughs)