When a humanoid was infected, their skin develop pale cracks across it, eventually overwhelming the body. In severe cases, the victim’s body would disintegrate into a cloud of dust when Praxeus infected the entire body. It could be spread when contact was made with open wounds and between species. Human astronaut Adam Lang became first and only known survivor of Praxeus.
The Doctor and her companions, joined by ex-police officer Jake, video blogger Gabriela, and medical researchers Suki and Amaru, investigate a new bacterium infecting people in Peru, Hong Kong, and Madagascar, discovering that infected people become covered by a crystalline substance before their bodies disintegrate. Birds in the local areas have also started acting aggressively towards humans.
The group finds Adam, Jake’s husband and an astronaut infected by the bacterium, in a Hong Kong lab. While rescuing him, they are attacked by humanoids in biohazard suits. Yaz and Gabriela remain behind to investigate further while the rest go to Suki’s Madagascar lab via TARDIS to study Adam. Yaz and Gabriela see another humanoid use a panel to teleport somewhere and they follow him, ending up in an alien-looking place.
The Doctor determines that the bacterium is drawn to microplastics that have saturated living beings, and that the birds’ natural enzymes are trying to fight it, causing them to become violent. As she generates an antidote, she realizes Suki’s lab is perfectly equipped for this job, and Suki reveals she is one of the remnants of an alien race ravaged by Praxeus, their name for the bacterium. When Yaz contacts the Doctor with their discovery, Suki teleports away. The birds kill Amaru and burst into the lab, and the Doctor and her group flee to the TARDIS, where she pilots it to Yaz’s location. Adam offers to be the test subject for the antidote, and the Doctor programs the TARDIS to synthesize more should it be successful.
When they arrive, the Doctor determines they are deep underwater in a shell of plastic made from the Indian Ocean garbage patch. They find Suki aboard her ship, and learn that her kind came to Earth, rich in plastics, to study Praxeus and create their own antidote. Suki demands the antidote from the Doctor but she reveals it works only on humans. Suki’s body, infected by Praxeus, is soon consumed and disintegrates. The Doctor finds the ship runs on organic fuel, and with Adam successfully cured, has her allies load the batch of antidote into the ship and sets it on course to explode in the atmosphere to distribute it. However, the autopilot fails, and Jake stays behind to pilot the ship on its course while the others retreat to the TARDIS. The Doctor uses the TARDIS to rescue Jake moments before the ship’s destruction. With the cure successfully spread, the Doctor drops off Jake, Adam and Gabriela and suggests they travel the world together.