After Doctor Who's giant reveal of a new Doctor plus Jack Harness, this week starts out relatively calm. The opening feels like it should be the beginning of Seven Worlds, One Planet until a spacecraft suddenly crashes through the atmosphere. As the astronaut, Adam Lang (Matthew McNulty), desperately tries to survive, somewhere else on the planet, an off-duty (currently inactive) police officer, DI Jake Willis (Warren Brown), is unsuccessfully attempting to keep law and order in his mum's shop. And in Peru, two travel bloggers, Jamila (Gabriela Toloi) and Gabriela (Joana Borja) of the vlog Two Girls Roaming have traveled to one of the world's most beautiful rivers. Only they discover it's become a plastic dumping ground, where the birds are unwell.
Graham: Well, excuse me, Inspector Morse, but I'm not the fantastist around here.
Jake knows Adam well enough to become emotional when he sees reports of his module crashing on TV. (We learn later they are married, but Jake's self-esteem problems have derailed things.) As he stares at the TV, he receives a mysterious text from Lang, begging for help, claiming he landed in Hong Kong. Meanwhile, Gabriela discovers Jamila is gone, having slept through her friend being attacked by birds in the night. But she's not alone. Ryan is in Peru, exploring what's gone wrong, collecting dead birds, and makes friends with a suspicious Gabriela.
Team TARDIS has split up to investigate this week's mystery. Graham and Yaz are in Hong Kong, tracking strange alien energy signals. There, they run into Jake and invite him to come along. The Doctor is in Madagascar, flagging down a pair of locals, Suki (Molly Harris) and Aramu (Thapelo Maropefela), for help. There's a body floating in the sea, in an American Naval Uniform, from a missing submarine. When he comes to, he says he's Zach Olson (Tristan de Beer), and the sub was under attack. But what's far more concerning is his body, rapidly growing over with scales until he explodes. He's not the only one with this sort of growth either. Ryan and Gabriela find Jamila, dead, with the same scaly growth. Ryan signals the Doctor, who TARDISes over to see it just before the body explodes.
Graham and Yaz hit the jackpot on the alien signals. Even better, the device that's giving off the readings has Adam attached to it. Yaz calls for the Doctor just as the aliens guarding the warehouse attack. She attempts to grab the device but fails. Jake and Graham grab Adam, and they make a run for it with Jake attacking the aliens to buy them time until the Doctor shows up. She ushers everyone to the TARDIS for a confab, but Yaz says no, demanding to go back for the device. Fans of Yaz have been complaining she's not getting her due, but this desire to run into danger without the Doctor is concerning. But Gabriela is raring to kick the ass of something, and aliens sound like a right topping adversary.
The Doctor heads back to Madagascar, where Suki and Aramu have turned up something else unusual. The birds are misbehaving in droves. Aramu promises to watch them while everyone else crowds into Suki's surprisingly well-equipped lab. As Ryan dissects the bird he picked up in Peru, the Doctor discovers Adam's been infected with an alien disease. It must be what the other two had because he's started to develop the same scales on his neck. Back in Hong Kong, Yaz determines the device is hooked to two others, one of which is in Madagascar. But at that moment, one of the aliens Jake injured crawls into a little pod in the corner and transports away.
Yaz convinces Gabriela the only sensible choice is to follow him. (Two Girls Roaming, indeed. Also, Yaz, do we need to talk?) The transport takes them to what Yaz thinks might be an alien colony. It's not clear where they are, but Yaz is chuffed to realize she found the missing submarine, and maybe her own alien planet.
Meanwhile, over in Madagascar, the birds are getting angry, attacking Aramu the way they did Jamila. Ryan discovers the birds are all sick with the alien disease, infected by eating plastic floating in the ocean. That's when the Doctor realizes how it's jumping to humans: the microplastics in the water. That makes this week's episode our second "Afterschool Special," this time a warning we're poisoning ourselves with plastics. But there's hope. (Because this is Doctor Who, and there's always hope.) Bringing the bird to Madagascar from Peru seems to have triggered something, because the bird is breaking down the virus, suggesting there could be a cure. Suki and the Doctor, who have been on the same page the whole way, are ready to create a "super virus" to kill this alien bacteria — until the Doctor calls Yaz and learns about the two signals from Hong Kong — one going to the Indian Ocean, and the other right here to this lab. Suki's face instantly says she's guilty.
The Doctor learns Suki's entire planet was infected with Praxeus. She and her crew were part of a team sent to find a cure. (Yaz didn't find an alien planet — sorry girl — she's just under the ocean, directly beneath a Great Plastic Garbage Patch.) This planet, being infested with plastic, was the perfect laboratory to introduce the disease and hopefully find the antidote. Suki believes the Doctor has done just that, going so far as to administer herself a dose, since she too is infected. But the Doctor doesn't even know if the cure works (Adam is currently her first human clinical trial). Besides, it was designed for humans. All Suki did was speed up the progression in herself, exploding before the Doctor can do anything.
Blessedly, Adam does get better, and the TARDIS has spat out a planet's worth of cure. The Doctor sets Suki's ship to take off and vent the antidote. But when the autopilot fails, Jake runs back and pilots it himself, proving he is worthy of saving the world, and his husband. It's a happy ending all 'round, with Gabriella even telling Ryan to come back and see her sometime. Roam if you want to....