Ed comes to sickbay, where Maggie is now staring and smiling and shooting water implacably at the windows. She is manic, and scary. Everybody freaks out all over the place, and Adelaide comms everybody to avoid all water supplies of any kind. "Don't drink the water. Don't even touch it. Not one drop." Andy smiles through the door at the Doctor, and he wonders if the things can talk. He points out to Adelaide that we're 60% water, which makes us the perfect host for whatever this is. Which thing precisely he does not know, and never will, because he is leaving. "Whatever's started here, I can't see it to the end. I can't."
Andy shoves himself up against the door, and the other one's shooting water at the edges. They're fusing the system, whatever that means, and run down a previous corridor, and come up against the robot, which the Doctor sonics so that he is now controlling the robot, which -- because the guy is strapped into a whole device for operating the robot -- means that he is also controlling Roman's body.
(As a person who has been foretelling the Doctor's descent into theocratic megalomania since the Martha season, that gave me the creeps. Luckily, this episode is all about the creeps. But there's something weird about a body controlling a robot that gets caught up in its own apparatus. Like a puppeteer being forced to dance around by some sudden tremendous upward force that's pulling on his fingertips, like a waterfall going up.)
So now the stupid robot -- as Adelaide and the Doctor climb aboard -- can go so fast that he lays tracks of fire like Marty McFly. It's sort of chicken/egg to wonder whether robots are naturally embarrassing, or if it's just the retarded shit they do. Maybe this is painful for Roman the dork in some way, I'm not sure why he's screaming. Maybe he's just that embarrassed by this. So then they end up at some doorway that's pretty much the same as the previous doorway, and the same things happen some more. Oh, and the whole episode the Doctor is bugging Adelaide about how they need bicycles even though that would have added prohibitive payload to the initial takeoff and is just basically empty silence that he is filling up with being repetitive, because as good as this episode is, there's not really a whole shitload of stuff that happens in it.
They go check out Maggie, who is staring at the window and splurting water boredly, and Adelaide tries to talk to her but she just stares, and the Doctor talks to her in quote Ancient North Martian, which gets her attention. He also notices that her eyes haven't got full-negative yet, not that she's looking human enough for comfort really, and they explain to the Doctor that they get all their water from a glacier underground in this crater they particularly chose for Bowie Base One. "If something was frozen down there," Ed grunts, "A viral life form, held in the ice for all those years..." Meanwhile, the Doctor notes around Maggie's blackened mouth the telltale signs of quote "some sort of fission" which apparently indicates that this viral thing "doesn't just hide in water, it creates water." And Yuri explains that now it, or she, or they, wants to go to watery old Earth. Where it will create even more water. Through "some sort of fission."