The Doctor and Bill in the TARDIS

The Doctor is showing Bill Potts the TARDIS when Nardole abruptly interrupts them. Suspecting that The Doctor is about to take Bill on a trip in his time machine he reminds The Doctor of his oath to stay on Earth and to protect the Vault. Despite Nardole's caution The Doctor sends him off to make some tea. The Doctor then lets Bill choose her first destination for a trip that will last for as long as it takes Nardole to boil the kettle.

The TARDIS takes them to the far future on an off-world Earth colony called Gliese 581 D. They find that the colony is empty except for millions of nanobots called the Vardy that maintain it, along with their robotic avatars that interact with the pair via emoji. The Emojibots give The Doctor and Bill badges. Each disc communicates the holder's expression, also through emoji. The Doctor and Bill try to put the disc on their chests but they automatically fly to their backs so preventing them seeing their own emotions being reported. The Doctor deduces that this is the only way to communicate with the Vardy.

As they look around the colony The Doctor is puzzled by the lack of any human presence and he becomes more sceptical at every turn. The Doctor suspects they have arrived before the full colony spaceship is due, but remains curious as to the absence of any type of pre-colony crew preparing for their arrival.

Nardole

When they enter a nursery The Doctor discovers an automated system spraying calcium-based fertiliser onto plants. The Doctor then finds a discarded necklace, which leads him to discover broken human skeletons being crushed to make the fertiliser. He realises that these must be the remains of the human party, sent in advance of the rest of the colonists. He deduces that the Vardy are perversely enforcing their programming that everyone should be kept happy by consuming anyone who experiences sadness, grief or other negative emotions.

An Emojibot then shows The Doctor the teary-eyed emoji on his mood badge, changing into the skull-faced emoji - the sign of death. Realising the danger they are in The Doctor warns Bill to keep smiling as they make their escape back to the safety of the TARDIS.

The TARDIS Arrives

Their The Doctor instructs Bill to remain in the TARDIS while he runs back to the city to destroy it and the Vardy before they can kill the incoming colonists expressing a 'childish impulse to blow it up'. In the TARDIS Bill wonders why The Doctor can't just call a 'helpline' to deal with the problem instead, but then notices the reassuring message on the TARDIS' police box door: 'Advice and assistance obtainable immediately' and realises that The Doctor is the person to provide this service.

Back in the city, The Doctor convinces an Emojibot that he is happy, but his happy-faced badge changes to show a light bulb shortly afterwards. The Doctor then realises that Bill has followed him back to the city. He explains to her that the entire city is made out of interlocked Vardy microbots. However, The Doctor believes that the centre of the city houses the spaceship in which the colonists first arrived.

The Doctor soon locates the spaceship and reveals to Bill that he plans to overload its reactor to destroy the Vardy before the arrival of the unsuspecting colonists. However, Bill discovers a young boy who leads her to a large number of hibernation chambers including some humans just awaking. On discovering this The Doctor realises that this is the colony's spaceship.

Looking Around

The pair review the spaceship's logs from the non-hibernating flight crew. They discover that the Vardy were programmed to help construct and operate the colony and make the colonists happy, using the emotion badges and avatars to communicate their emotional state. When one of the flight crew died naturally, it created grief amongst the crew that the avatars were not programmed to register. The Vardy took this as a sign of disease, and killed those who displayed unhappiness. This however, created a 'grief tsunami' which rapidly wiped out the entire flight crew, and will likely wipe out all the waking colonists, who are the last survivors of earth, when they discover what has happened.

On awakening The Doctor informs the colonists on what has happened and the danger they are in. Despite The Doctor's warning the colonists decide that they must leave the spaceship and combat the Vardy to protect their hibernating crew.

Failing to dissuade any fighting, The Doctor solves the problem of the deadly robots by rebooting the system to a point before they saw grief as a disease. The Doctor also advises the Emojibots that they could make a fortune charging the humans rent as an added bonus for keeping them alive.

The Doctor and Bill then return to the TARDIS which returns them to Earth. But instead of taking them back to The Doctor's study they find themselves on the frozen Thames as an elephant looks on...