Bill Potts is learning that life with the Doctor can be an adventure, and kind of dangerous, sometimes. This week, she learns it can also be deadly, as she heads to an event with a shocking secret.

SPOILERS BELOW

They are at the Frost Fair of 1814, where the Thames has frozen over. Bill is a bit worried about being noticed, but learns the TARDIS has a wardrobe for every era. She admits she’s kind of “low-key” in love with it.

They soon find out that under the ice, though, is a strange sea monster that is eating people. Bill sees Spider (Austin Taylor), a street urchin, being eaten, while the Doctor barely saves his friend Kitty (Asiatu Koroma).

Bill wonders why the Doctor can’t save Spider, although his fate was sealed. He admits that sometimes people do die when he is around, and he has killed, too, but he has to move on. Bill is shocked by this side of the Doctor’s life, but realizes she has to move on to save others from the monster.

They decide to be “bait” by walking in diving gear above the ice, and hoping the monster will eventually notice them. It’s a sea monster that is chained to one spot. So who could do such a thing?

Thanks to some fast talking, they find out that someone is dredging the river and getting something that burns better than coal, which is good news for the local steel mills. Yes, mills are using dung from the creature as fuel.

It’s traced back to Lord Sutcliffe (Nicholas Burns), who knew about the monster for quite a while. He apparently runs the Frost Fairs to get victims for the creature, and then he gets the dung for his mills. He considers that progress.

The Doctor doesn’t, saying that it’s knowing the value of a person that is progress. He points out Sutcliffe’s value is equal to the street urchins and the unknown people who he feeds to the monster. Sadly, he doesn’t see it that way, and plans to blow up the fair to silence the Doctor and Bill and feed the creature. At least she knows who has the real compassion here.

After they both get free, the Doctor poses a tough question to Bill:  what should be done with the creature? Should it be killed, since it could eat others once the river thaws? Her answer is quick:  Save it.

After Bill tries to get the people away by claiming the ice is melting, Sutcliffe decides to set off the bomb early. It goes off…and frees the creature. It surfaces to crack the ice, and Sutcliffe meets a justifiable end. The Doctor thinks the creature will be headed north, but Bill wonders if it might stop off at Greenland. He promises to check.

So what about Kitty and the urchins? Thanks to some quick forgery, the Doctor names Kitty’s friend, Perry, as Sutcliffe’s long-lost heir. When they get back to 2017, they learn the plan worked. However, no one remembered seeing the big creature that swam out to sea. The Doctor suspects some must have thought it was due to bad ale.

It was an interesting couple of episodes for the Doctor and Bill, and fans have been really enjoying their chemistry together. It’s a crime it’s for one season, but let’s hope Pearl Mackie gets another role soon…and maybe a return to the show a couple of seasons down the road.

While the Doctor and Bill were busy, Nardole (Matt Lucas) thinks they were still in the office while he’s brewing tea. He reminds the Doctor not to go off-world, which technically he didn’t.

Nardole is later found at the vault that he and the Doctor are supposed to guard. And someone…or something…is knocking from within, but Nardole says whatever’s in there isn’t coming out.

Yet.

Next week, Bill and her friends are having landlord problems after moving into a mysterious house.