Baltazar: The Doctor's going to find me the Infinite. Going to find me my hearts desire. And then I will have my revenge.
Martha: Are we going on a quest then?
The Doctor: No. The Infinite stays lost, nothing more than a legend. As the last Custodian to the secrets of the universe it's my duty, to see it stays that way.
The Doctor: Let's just say that there are things out there in space, Martha, things that pre-date our reality. Relics from the dark times.
Martha: Oh, a fairy tale.
The Doctor: Oh, far, far more than that. There was a time when the universe was so much smaller than it is now. A darker, older time of chaos. Creatures like the Racnoss, the Nestenes and the Great Vampires rampaged through the Void.
Martha: And this Infinite, is one of those relics from the dark time?
The Doctor: It's said that if anyone were to find the Infinite they would receive their hearts desire. And frankly, that's quite a thing to receive.
Caw: He [Baltazar] seeks his hearts desire. And knows how to find it.
The Doctor: He does?
Caw: The Infinite.
The Doctor: Oh, that's just a legend.
Martha: What is?
Caw: Baltazar thinks different. Thinks he knows how to find it.
Martha: Find what?
The Doctor: The Infinite.
Martha: Yeah, I got that. But what exactly is the Infinite?
Caw: (on Baltazar being released from prison) He'll be coming for Caw. Caw sold him out. For a bar of gold. Or three.
Martha: What do you need gold for?
The Doctor: He eats it.
Martha: (gazing upon a planet populated by giant birds) I've died and gone to Bill Oddie heaven.
The Doctor: Yes! Six hundred and seventy million miles an hour...and a bit. Rock on!
Martha: You're just a boy racer, aren't you?