Gabriela and Jamila

The Doctor and her three travelling companions, Graham O'Brien, Ryan Sinclair and Yasmin Khan, investigate a strange new bacterium infecting people around the globe.

In Peru, Ryan meets travel vlogger Gabriela, who is looking for her missing companion Jamila. When they find the odd-looking corpse of a bird, Ryan takes it in his backpack. They later come to an abandoned hospital, where they encounter the dying Jamila infected with a crystalline substance, who then disintegrates in front of their eyes.

In Hong Kong, Graham and Yasmin meet up with ex-police officer Jake, whose husband Adam Lang, an astronaut, has crash-landed in the Indian Ocean, but seems to have sent a text message from the city asking for Jake’s help. While investigating a derelict warehouse, they find some alien looking technology including a strange translucent panel in an alien language which Yasmin tries to take, before they are attacked by two humanoid figures in gas masks. Jake handily defeats the two figures in close combat after shooting them with one of their laser rifles.

Jake Willis

In Madagascar, The Doctor finds US Navy submarine officer Zach Olsen washed ashore, and gets help from medical researchers Suki and Amaru to bring him to safety, but the naval officer reveals that his submarine was attacked and he was also infected. Zach Olsen promptly succumbs to the bacterium. The Doctor, Suki and Amaru then witnesses the same odd behaviour of the birds that Ryan saw in Peru.

The Doctor collects Ryan and Gabriela via the TARDIS, and then lands in Hong Kong to recover the others. Yasmin though insists on staying behind so that she can try and retrieve the panel. Gabriela offers to help. As the others return to Madagascar to take care of Adam, Yasmin and Gabriela return to the alien lab but find another humanoid in a gasmask. He uses a separate panel and teleports away, and Yasmin and Gabriela decide to follow, finding themselves in what appears to be an alien environment.

Graham and Yasmin in Hong Kong

In Madagascar, Jake is glad to see Adam, but sees that Adam has become infected as well. The Doctor studies the bacterium, along with the bird corpse Ryan had collected from Peru. She finds that the bacterium attaches easily to plastics, including microplastics; the natural enzymes of the birds though are trying to prevent the alien bacterium from spreading. The Doctor surmises a better antidote must be made to prevent the birds from spreading it across the whole planet.

While working in Suki's lab The Doctor wonders why it is so well-equipped for its size and location, and suspects Suki may be hiding something. Realising that her cover is blown Suki reveals that she belongs to the remnants of an alien race affected by the bacterium, which they have called Praxeus. Suki's race have been deliberately spreading the bacterium on Earth, rich in plastics, to investigate its effects and find an antidote.

When Yasmin contacts The Doctor about her location, Suki suddenly uses a device to teleport away, absconding with the formula for the antidote. Outside, Amaru is attacked by numerous birds, which then burst into Suki’s lab. The Doctor hurries Graham, Jake, Adam, and Ryan into the TARDIS and sets course for Yasmin’s location. En-route, she uses what she had learned to create an antidote for Praxeus, and Adam offers to test it so that the TARDIS can make more, against Jake's concerns.

Suki and Amaru

When the TARDIS lands near Yasmin’s location, The Doctor discovers that they are deep under the Indian Ocean, inside an alien shell made from accumulated plastic waste. The Doctor, Ryan, Graham, Yasmin, Jake, Adam, and Gabriela encounter a wounded masked figure, but he succumbs to Praxeus. They then find Suki tending to her unpowered spaceship. Suki reveals she was able to put off succumbing to Praxeus because her species has a different metabolism, and that her spaceship uses organic fuel cells. Suki has transmitted the formula for the antidote to her species and concocted and taken The Doctor's human-only antidote, but it leaves Suki more vulnerable to Praxeus, which quickly overtakes her body and kills her.

The Doctor finds out that her antidote has worked on Adam, and loads the supply into the fuel cells. She repairs Suki’s spaceship and sets it to fly off and explode in the stratosphere so as to distribute the antivirus, but the autopilot malfunctions.

The Doctor, Graham, Yasmin, Ryan, Gabriela, and Adam rush back to the TARDIS. But once inside they find that Jake has stayed behind so as to pilot the spaceship to make sure it reaches its destination. Knowing that Jake will not survive unless she intervenes The Doctor is able to materialise the TARDIS around Jake and rescue him a millisecond before detonation.

After assuring the antivirus has properly spread around the Earth, via the jet stream, The Doctor drops off Adam, Jake, and Gabriela in Madagascar and encourages them to travel the world together. The Doctor and her three travelling companions then depart in the TARDIS.