He watches Adelaide remember. For us, it was only a year ago, when the planets were stolen and the skies replaced. Her father left her hiding in an attic, and went looking for her mother, and they both died. They were never found; this part makes the Doctor saddest. But while she was up there -- in the midst of extinction, flying just a little bit above it; waiting for her family that would never return -- she looked through a skylight at a Dalek, who stared down at her. Down on the streets there was panic, and burning, but up in the sky an alien inspected her, and looked right into her. And she looked up at it, and -- she nearly smiles, now -- it simply went away.
"I knew, that night, I knew I would follow it." But not for revenge, the Doctor can see. She still has tears in her eyes when she asks what the point of that would be.
That's what makes her remarkable, and that's how she'll create history: "Imagine it, Adelaide. If you began a journey that takes the human race all the way out to the stars. It begins with you. And then your granddaughter, you inspire her. So that in thirty years Susie Fontana Brooke is the pilot of the first lightspeed ship to Proxima Centauri. And then everywhere. With her children, and her children's children forging the way, to the Dragon Star, the Celestial Belt of the Winter Queen, the Map of the Water Snake Wormholes. One day a Brooke will fall in love with a Tandonian prince, that's the start of a whole new species. But everything starts with you, Adelaide. From fifty years ago, to right here, today. Imagine."
She does. And she asks him who he is, and why he is telling her, and the Doctor almost weeps as he gives the only possible answer he can give, something so clear she could have figured it out for herself: As consolation.
The computer beeps, spitting out their answer: The #3 water filter, Andy reported, busted earlier today. And since the water isn't cycled out of the Biodome for a week, none of them are infected. She radios ahead to Ed as she's running; he's the one piloting their escape shuttle, and it's waking up outside. Everyone's excited, giddy. Even Adelaide. She sends him off after his ship, proud of herself for saving her people. "I know what this moment is: It's the moment we escape!"
The Doctor lingers, unsure and even more afraid, as they rush around. He realizes Tarak is climbing up onto the roof of the dome, which nobody really notices because they're all busy running around and substituting weights in the cold equations, wondering how much food they can safely leave behind, until Tarak and Andy are right over them. Right above them, on the roof. Dripping and flooding great jets of water against every seal and lock. They don't need oxygen, because they breathe water, and they don't need spacesuits because quote "they've got that internal fission," which means the crew is still technically safe but the Flood can last forever and have unlimited amounts of water to use against them, so they are not actually safe. Adelaide finally yells at everybody to pull it together and sends Ed back to the shuttle with a smile.