There have been attacks on intelligence agents around the world. The head of MI-6, called C (Stephen Fry), calls in the Doctor (Jodie Whittaker), and her companions Graham (Bradley Walsh), Ryan (Tosin Cole), and Yasmin (Mandip Gill) to investigate. The DNA of the victims has been altered into something that seems extraterrestrial. The CEO of a search engine company called VOR seems involved—Daniel Barton (Lenny Henry). The Doctor contacts agent O (Sacha Dawan) who was in charge of extraterrestrial investigations until C fired him. Suddenly, C is dead, killed by mysterious creatures, though the Doctor and companions manage to escape.
Yaz and Ryan investigate Barton (recorded as 93% human) at his birthday party. Graham and the Doctor find O in the Australian outback. They all encounter luminescent aliens who seem to be working with Barton. The Doctor captures one in Australia, who tells her that they intend to occupy the universe. Yaz is attacked by one while sneaking into VOR headquarters with Ryan and is transported to a strange dimension. But they are finally united in O’s base in Australia.
O joins the investigation and they investigate Barton at the party. The Doctor confronts Barton. He denies everything and drives off. The Doctor and company pursue him on motorbikes to his private jet, where O is revealed to be the Doctor’s old foe, The Master, who has been in cahoots with the aliens all along. Barton then disappears and leaves a bomb behind on the plane. It goes off and the plane goes into a nosedive. The Master teleports away to his TARDIS. The Doctor vanishes and lands in the same environment as Yaz, and the rest are trapped in the crashing plane.