Bill’s back in the TARDIS with a million questions. How does it work? Why are the seats where they are? How much did it cost? When The Doctor says he stole the TARDIS, Bill actually looks impressed.
Their chatter is interrupted by Nardole who suspects that The Doctor might be intending to take a trip and is not happy. The Doctor insists he’s not going anyway and instructs Nardole to put on the kettle for tea. Which Nardole agrees to do but he refuses to make a cup for Bill. Not really a shock that the Doctor has no intentions of going straight back to his office. After all, the TARDIS is a time machine. They can take a trip, and come back to the same time and Nardole will have no idea they left.
Bill makes the choice to go to the future. She wants to see if the future is happy.
Cut to a mystery planet covered in fields of what appears to be wheat. A uniformed worker woman and something she calls Vardies, are crossing the fields towards a bright white city. A female voice on her radio (which appears to be implanted in her earlobe) tells her not to return but she’s hungry so she ignores the request. On arriving back inside the city, the other woman tells her to keep smiling while telling her that several other people are dead. The worker woman is not surprisingly saddened by the news, which is reflected on an emoji button on her clothing. Then the swarming Vardies attack her and she’s reduced to nothing but bones, as is the other woman while a short humanoidish robot watches.
Bill and The Doctor arrive in the TARDIS, landing several yards from the city. As they cross the fields, Bill quizzes The Doctor about why he made the TARDIS look like a police box, but he ignores the question. Instead he tells Bill that they are on an Earth colony that was said to have discovered the secret to happiness.
Bill asks The Doctor about his oath but he’s very vague in giving her an answer, deflecting to comments about their surroundings etc. As they wander through the city they are upgraded to the cities audio system and then receive a pair of the emoji mood buttons from one of the mini people looking robots. One that actually speaks emoji to Bill’s delight (and The Doctor’s distain). The little robot brings them food that resembles a cube of gelatin but tastes like meat and such. When Bill notices that the Doctor was given two pieces, he tosses out the detail that he has two hearts so the robots likely think he’s two people.
The Doctor is perplexed by the lack of people until Bill makes a comment about how it feels like the Student Union before they open for the day. The Doctor realizes that they have arrived on the colony before the colonists have arrived, which is why it’s empty. Bill and the Doctor continue their exploration, wandering through gardens to a greenhouse. Things turn dark when they find several skulls and realizes that the crops are being fertilized by pulverized human bones.
Initially, Bill and the Doctor make a run for it, unconcerned about why the humans were killed. That is until they are cornered by several of the emoji bots. The Doctor sorts out that the reason the robots are coming after them at that time is that they are no longer happy. He and Bill manage to fake being happy and cheerful long enough to get around the robots.
Once they reach the TARDIS, The Doctors tells Bill to stay put while he goes and blows up the city before the colonists arrive and are killed. Bill is confused about why The Doctor insists on dealing with the issue until she notices the sign on the door to the TARDIS and the line about “advice and assistance”. It clicks in her mind that The Doctor sees himself as that assistance. And she goes back to help him.
They quickly figure out that the city is made up of Vardies, formed around the ship that brought them to the planet. Once they find the ship, the Doctor leaves Bill at the site of a ship schematic to guide him to the engine room so he can blow up the engine and the ship/city with it. It takes Bill several minutes before she remembers she has a smart phone and could just take a photo of the map, or that The Doctor had the map memorized and it was a ploy to keep her safe. Bill goes to join the Doctor but is sidetracked when she discovers a room with the body of an elderly woman inside. At the woman’s feet is a book that appears to hold all of Earth’s history from its art to all the various wars. Bill figures out that something bad must have happened on Earth, which The Doctor somewhat confirms. As Bill goes to help The Doctor in the engine room she stumbles onto a living human boy.
The boy takes The Doctor and Bill to a vast chamber and the Doctor figures out that it contains the last survivors from Earth. Survivors who are now waking about, into a world full of microbots that will kill them if they have a sad day. The Doctor has to figure out why it happened at all so he can stop it from repeating. Bill shows him the body and they figure out that she died of old age and it confused the Vardies who misread grief as a super bad condition and destroyed those who were grieving, expanding outward until everyone from the first wake up group was dead. The Doctor attempts to explain to the newly awakened colonists what’s happened but the colonists only hear that the Vardie killed their friends and are enraged. They gather guns and set out to destroy the robots.
Only the Vardie have become self aware and they fight back. The Doctor uses a destroyed emoji bot and his sonic screwdriver to cause an massive system reboot. The reboot wipes the Vardie’s memory and all their happiness programming. The Doctor helps to negotiate peace and a rent agreement for the colonists to stay in the Vardie city and he and Bill head home. Only they miss their target in time and end up somewhen else. Oops.