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."
None of this makes a goddamn bit of sense. Not a lick of sense does any of that make. Not even on the surface level where I prefer to live does that make sense. Not in a diagram-this-sentence way does it make sense. Maybe that whole part would have been less confusing if they'd just straight up said this stuff, or had one of the racial stereotypes ask an Excellent Question and the Doctor could just have at it, or even called a time-out to be like, "Here is the shitty science in your science fiction," with charts and graphs, instead of pretending it was this conversation where like one thought logically followed another.
Ed reminds Adelaide, in a whispered aside, that this unknown but quickly spreading infection "demands Action Procedure One," and Adelaide is like, "I know that," and they flirt about how she doesn't need reminding about anything but that he's good for something every now and then, and she actually smiles, and then the Doctor interrupts to ask Excellently whether Action One does in fact mean evacuation, and Captain Brooke. declares Action One, immediate evac.
The rest of the crew runs around packing everything up, and Adelaide gives them unrealistically tight deadlines to get things ready to go, and the two major water guys are still off in the biodome tunnel, and Maggie and the Doctor are both staring all around at this stuff, because he knows this Action One isn't getting off the ground, and he should really be leaving. He is quiet but you can tell he doesn't want to leave, like at all, and Adelaide gives him his spacesuit back, but his stupid head's been working once again.