The Doctor (Peter Capaldi) allows Bill Potts (Pearl Mackie) to choose the destination on her first trip in the TARDIS, even through Nardole (Matt Lucas) has his doubts. They land on an Earth colony in the far future, which they discover to be uninhabited, except for millions of nanobots that continually repair it, and robots that seem to communicate with face emojis. The Doctor thinks they have arrived too soon, and the humans are still on their way in colony ships but wonder why there is no crew to prepare for their arrival. They discover human bones being used as fertilizer. If the visitors do not seem to be happy, they will be consumed. They must escape, smiling at all times.

The Doctor vows to destroy the nanobots—called the Vardy—before the unsuspecting colonists show up. He finds the ship that brought the now-dead preparation crew and plans to overload the reactor. But Bill runs into a young boy who leads her to row upon row of hibernation chambers. The Doctor realizes that this is the colony ship, already arrived, and the passengers are awakening.

They review the ship’s logs of the non-hibernating flight crew. The Vardy were programmed to build and operate the colony and make everybody happy. Someone died and the resulting grief was not something the Vardy could cope with. They believed that unhappiness was a disease. As the body count rose, more and more humans were overcome with grief and the flight crew was wiped out. The Vardy are becoming sentient and have a sense of self-defence. The Doctor manages to reset them to before they saw sadness as a disease. He offers to negotiate between the humans and the indigenous species—the Vardy. The time-travellers leave and return to Earth, where they find themselves standing on the frozen Thames looking at an elephant.

The colony ship is called Erewhon, after the 19th Century dystopian novel by Samuel Butler. Erewhon is nowhere backwards, sort of. The story was well-received, particularly the growing friendship between the Doctor and Bill. The Vardies are named after Professor Andrew Vardy at Memorial University, Newfoundland, an expert in swarm robotics. He helped write a science-fiction story on the subject. The Doctor quotes David Bowie: “I’m happy, hope you’re happy too.” He is a big Bowie fan. The startling architecture was filmed at the City of Arts and Sciences in Valencia, Spain. Bill says the Doctor runs like a penguin with his arse on fire. He says there are many Scotlands in the Universe, and all of them want independence. The Emojibots somehow manage to be cute and creepy at the same time. Bill is eminently likable, Nardole is a fussbudget of the first water, and the Doctor seems to feel like a kid again.