"So I'm the Doctor, and you are?"
The very first off-world colonists in Earth history, is who they are: He's standing on Bowie Base One, established the First of July in the year of Our Lord 2058. The Base has been up and running for seventeen months. It all clicks and the Doctor laughs. "My head is so stupid, you're Captain Adelaide Brooke! Died 2059. And Ed! You're Deputy Edward Gold." He points them out: Tarak Ital, MD, and Nurse Yuri Kerenski. Senior Technician Steffi Ehrlich and Junior Technician Roman Groom. Geologist Mia Bennett, who is only 27 years old. Who was only 27 years old, today, the Twenty-First day of November in the year of Our Lord 2059: The day she died.
The Doctor apologizes -- "I'm sorry with all of my hearts" -- but it's one of those fixed moments, like Pompeii: "One of those very rare times when I've got no choice." It was the Mars disaster. The kind of tragedy that brings us closer to the universe. He shakes their hands, lovingly, like a dumbstruck fan, ready to go, even as they're inviting the gardeners to come see the strange man before he vanishes again. But it's already started.
There's a strange growling on the comms and they can't get the gardens' cameras going; an external shot shows the lights shutting down, one by one. Adelaide orders the Doctor to accompany her out there, to see what's going on; he swears he's leaving and she has one of them lock up his spacesuit. "This started as soon as you arrived, so you're not going anywhere. Except with me." He accedes, worried.
On the walk Adelaide interrogates him about his strangeness, his mysterious change of heart, the way he focused on their names and the date, on Mia's age. Over the comms the Doctor makes friends with the unbalanced emo kid, who pilots the stupid robot along with them as they go; he asks Captain Adelaide if it was worth it. "We've got excellent results from the soil analysis," she answers, confused, but he means something larger. He means something you can't possibly say in words, because he needs to believe it: "All of it. Because they say you sacrificed everything. Devoted your whole life, to get here." Her back straightens, strong: "It's been chaos back home. Forty long years. The climate, the ozone, the oil apocalypse. We almost reached extinction. Then to fly above that... To stand on a world with no smoke, where the only straight line is the sunlight...? Yes. It's worth it." The Doctor smiles, and loves her: "That's the Adelaide Brooke I always wanted to meet. The woman with starlight in her soul!"