(I was out of town for over a week, missing both “Mummy” and “Flatline,” which is why this is later than usual. I wrote it not having seen “Flatline.”)
It’s “one last trip” for Clara in the TARDIS, and the Doctor takes her to the Orient Express - in space. Unsurprisingly, the trip turns deadly when, one by one, the passengers and crew die at the hand of a mummy only the condemned can see, exactly 66 seconds before his or her death. With Clara trapped in a storeroom, the Doctor tries to rally other passengers to determine what the mummy is and how to defeat it, using each victim as an opportunity to learn more.
The episode provided the chance for the cast to appear in 1920s period costume. Clara is cute in a flapper dress. The Doctor takes a cigarette case out of his coat pocket, opens the case, and slides it to a gentleman… only to reveal Jelly Babies strapped in the case. Very cute. Sure, we’ve seen the “historical model of transportation in space” bit before, with the Titanic in “Voyage of the Damned,” and we’ve seen 1920s costume in “The Unicorn and the Wasp,” and we’ve seen murderous mummies before in “Pyramids of Mars,” but combining these elements with the cranky Twelfth Doctor was a delight. The revelation that the mummy is actually a soldier with defective gear from a long-ended war fits in with the season-long arc of the Doctor’s dislike for soldiers (including Danny Pink).
We also see Clara’s character continue to develop, and in a surprising way. During the episode, the Doctor asks that she lie to Masie, a passenger, to induce her to come to the Doctor’s makeshift lab. While Clara appears very uncomfortable doing so, by the end of the episode she has become far more comfortable lying. After she talks with Danny - who asks her if she’s had her last trip and has left the Doctor for good - she first lies to Danny, saying she has, and then lies to the Doctor, saying that Danny is fine with her continuing to travel in the TARDIS. One suspects those lies will boomerang back to Clara later in the season.
The Doctor also changes a bit, explaining to Clara at the end that he didn’t know if he could save Masie and was unwilling to give her false hope. While we see his practical side - the Doctor would dispassionately use as many deaths as necessary to understand and stop the mummy - we also get a hint that he still cares about saving lives.