Doctor: Peter Capaldi (Twelfth Doctor)
Pearl Mackie (Bill Potts)
Matt Lucas (Nardole)
Kiran L. Dadlani – Kezzia
Mina Anwar – Goodthing
Kaizer Akhtar – Praiseworthy
Ralf Little – Steadfast
Kalungi Ssebandeke – Nate
Kiran Shah – Emojibot 1
Craig Garner – Emojibot 2
Directed by Lawrence Gough
Written by Frank Cottrell-Boyce
Produced by Peter Bennett
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 10
Length 46 minutes
Originally broadcast 22 April 2017
Despite Nardole's caution, the Doctor allows Bill to select her first destination for a trip in the TARDIS. They land in the far future on an off-world Earth colony.[a] The colony is empty except for millions of nanobots called the Vardy that maintain it, along with robotic avatars that interact with the pair via emoji.[b] The robots give the Doctor and Bill badges which report their emotions also through emoji—seemingly the only way to communicate with the Vardy.
The Doctor suspects they have arrived before the full colony ship is due, but remains curious as to the absence of any type of pre-colony crew preparing for their arrival. They discover the bones of these humans being used for fertiliser. The Doctor realises that if the Vardy sense they are not happy, the nanobots will consume them to their skeletons, and warns Bill to keep smiling as they escape. The Doctor locates the ship that brought the pre-colony crew to the planet, planning on overloading 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; the Doctor realises that this is the colony ship.
The pair review the ship's logs from the non-hibernating flight crew. The Vardy were programmed to help construct and operate the colony and make the colonists happy, monitoring the emotional state of the colonists through the emotion badges and avatars. 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 created a "grief tsunami" which rapidly wiped out the flight crew, and will likely wipe out the waking colonists when they discover what happened.
The colonists believe they must combat the Vardy to protect their hibernating crew, while the Vardy start to show the beginnings of sentience through self-defence. Failing to dissuade any fighting, the Doctor resolves the situation by "resetting" the Vardy to their original state, before they saw grief as a disease. The Doctor offers to help negotiate between the colonists and Vardy to avoid the same mistake. Later, the Doctor and Bill return to Earth, but find themselves on the frozen Thames as an elephant looks on.