This week’s episode—Oxygen—is the first to show Bill in serious danger, courtesy of a malfunctioning spacesuit that nearly kills her and also, ultimately, saves her from dying: a dichotomy that seems to point to the uneasy relationship some people have with capitalism—a theme that is explored here. And again racism is challenged. I’m reminded of classic Star Trek episodes that score an A-plus for science fiction, while still finding time to scrutinize modern-day prejudices.

Not surprising, then, that the Doctor’s first line is “space, the final frontier.” Although, Oxygen—which too easily slips into on-the-nose dialogue in an otherwise solid Doctor Who episode that expertly knits chunks of knotty science into the plot—probably deserves a B-minus at best.

An opening monologue from the 12th Doctor (Peter Capaldi) appears to hint at his imminent regeneration. “… Final,” he says, “because it wants to kill us—sometimes we forget that. Start taking it all for granted. The suits, the ships, the little bubbles of safety as they protect us from the void.” It also tells the story of what is about to unfold.

It helps that the Doctor, in season 10, is a lecturer of everything and anything on Earth. While he is meant to be teaching a lesson on crop rotation he instead gives the students, and the viewers, a tutorial in space vacuums (“Don’t hold your breath or your lungs will explode!”). Which is handy, since that’s something he faces after responding to a distress call in deep space.

At last, Nardole (Matt Lucas) comes along for the ride with the Doctor and Bill (Pearl Mackie). And it means that the thing/monster/person trapped inside the mysterious vault they are suppose to be guarding has been left alone—unwatched. Is this deliberate? At the end of Knock Knock, the Doctor paid a visit to the piano-playing guest who has a penchant for grizzly haunted-house stories (don’t we all?). So, he knows who’s inside, even if the rest of us are still waiting for the big reveal. Has he left the vault unattended for a reason? Or is he getting a bit slapdash with his babysitting duties?