Written by Pete McTighe and Chris Chibnall. This episode first aired February 2, 2020.
Above the Indian Ocean, British astronaut Adam Lang's Soyuz descent capsule malfunctions suddenly, causing him to go missing. In Peru, travelling vloggers Gabriela and Jamila camp by a beautiful river that has become a polluted dump site, but when Jamila wakes up in the middle of the night, she's suddenly attacked by a flock of birds. When Gabriela wakes up and finds her friend missing and dead birds lying about, she tries to touch one... only for Ryan to suddenly turn up and warn her not to do so, picking up the bird's corpse in the process. In the UK, Adam's troubled ex-policeman husband, Jake Willis, discovers that he is missing, presumed dead, just before receiving several text messages from Adam asking for help and placing him in Hong Kong. When Jake arrives, he tries to kick down the door to the building where Adam apparently is, only to be met by Yaz and Graham, who have seen strange energy readings and have the skeleton keys to enter. In Madagascar, scientist Suki Cheng is annoyed when she is yet again beaten to work by her colleague, Amaru, when they are called to help rescue a man floating in the surf by the Doctor. The man, survivor of a missing US submarine, panics about something that got inside the vessel before his body is taken over by white scale-like growths and disintegrates.
All of these events, around the world, have a connection... but what is it? A check on social media leads Ryan and Gabriela to an emergency call and a hospital where Jamila was taken. But the hospital is eerily empty, and her body, under a sheet, has more of the white growths. Ryan calls the Doctor.
The Doctor arrives in the TARDIS in time to witness Jamila's infection spread across her face and cause her to explode. The Doctor, Ryan, and Gabriela set off for Hong Kong and meet with Graham, Yaz and Jake, who has just kicked down the door to the room where the energy readings are emanating from and discover Adam barely conscious in a strange alien lab. Yaz notices a translucent panel with alien language next to Adam and recognizes its value. The Doctor, Ryan, Gabriela, Graham, Yaz and Jake evacuate Adam, but two humanoid figures in gas masks gang up on them. Jake subdues them in close combat and shoots them with one of their laser rifles.
Yaz insists to the Doctor that she needs the translucent alien panel, and has to track the aliens down. Gabriela agrees to stay behind in Hong Kong, also to the Doctor's reluctance. The Doctor agrees to let them go for one hour, while she travels to Madagascar to take care of Adam. Yaz and Gabriela return to the alien lab and try to recover the panel, but find another humanoid in a gas mask. He teleports away. Yaz and Gabriela decide to follow him to what appears to be an alien environment of green lights and plastic waste held together by fishing nets, where they are confronted with the mast of the missing submarine. Yaz wonders aloud whether she has discovered an alien planet.
In Madagascar, the Doctor and company discover that the growths have spread on Adam's face. The Doctor asks Suki if her lab has enough advanced medical equipment, up to a gene sequencer, and Suki confirms it. The Doctor asks Amaru to stay out in front and watch the birds while she studies the growths and Ryan dissects the bird corpse he brought along, which is full of plastic. Meanwhile, Graham assures Adam he knows how to take care of him with an IV drip, having been through cancer treatment. As they wait outside on the seashore for Adam to heal, Graham comforts Jake, who reveals he has an inferiority complex from being married to Adam; Jake also hates travelling abroad and is afraid to commit, which is why he wasn't present at Adam's launch. The Doctor finds the growths and the odd behaviour of the birds stem from an alien bacterium which attaches easily to plastics, including microplastics; the natural enzymes of the birds are trying to prevent the alien bacterium from spreading.
The Doctor figures that the bird enzymes can be used to engineer a cure for the bacterium. But after she's gotten a rudimentary antidote synthesized, and as she wonders why a lab of such size and high-tech would be in such a location as rural Madagascar, Suki reveals herself: she's been researching the disease, called Praxeus, the entire time, and now that a cure has been found, she can take the data. The infected birds attack the lab, killing Amaru outside before heading inside, as per Suki's warning that Praxeus knows when people are trying to cure it and will fight back. Suki teleports away, leaving the Doctor, Graham, Ryan and Jake, hauling along Adam, to run for the TARDIS and ward off the blackbirds.
The Doctor rigs up a way to keep Adam alive within the TARDIS console room, and has the TARDIS make an improved antidote. Adam and Jake briefly argue over whether Adam should volunteer as a test subject, but Jake relents. They share a moment together and begin to reconcile.
The Doctor locks onto Yaz's coordinates and lands the TARDIS in the plastic landscape. Yaz reunites with the Doctor, only for the Doctor to disappoint her by revealing they are underneath the Indian Ocean garbage patch, laden with plastic. The Doctor, Yaz, Graham, Ryan and Gabriela find the wounded humanoid and unmask him, triggering his death from Praxeus. The five of them find Suki inspecting the engines of a small unpowered alien space ship. Suki reveals the ship is hers, and her race are almost dead from Praxeus, but have been able to hold off Praxeus because of their different metabolism. However, they cannot continue to thrive without finding an antidote, and had searched three galaxies before landing upon the perfect plastic-polluted planet on which to investigate Praxeus' effects: Earth.
The Doctor chews Suki out for this, but Suki tells her she has already transmitted the formula for the antidote to the rest of her species. The Doctor reveals the antidote is human-only and unlikely to work on other species, leaving Suki, who has tested the antidote on herself, more vulnerable to Praxeus. Suki promptly disintegrates.
The Doctor then notices Adam has grown healthy, proving the antidote worked, and that the ship has organic fuel cells. She realizes the fuel cells can be used to amplify the effects of the antidote and distribute it farther out. The Doctor, Yaz, Ryan, Graham, Jake, Adam and Gabriela repair the ship and set it to fly into the stratosphere to explode and help destroy Praxeus. They escape the ship, only to find that the autopilot is not working and Jake has stayed behind to guide the ship to its final destination. Adam pleads to Jake not to sacrifice himself. Jake points out that he wants to do more to save the billions of lives on Earth. The Doctor rushes to materialize the TARDIS around Jake, though, saving him the millisecond before the ship explodes.
The Doctor drops Adam, Jake and Gabriela off at the lab in Madagascar. Adam comments that he always wanted a honeymoon, and Jake relents, taking in the view in the meantime. The Doctor and Ryan encourage Gabriela to look for a new travel companion in Jake and Adam.
- 555: Or at least the British equivalent, 07700 900xxx. Unfortunately, the producers made a small mistake as the phone number displayed is +852 7700 900xxx, which is a British phone number preceded by the IDD code for Hong Kong. In Real Life, IDD codes are tied to the phone number and don't change depending on where you are.
- Audience Surrogate: The word "pathogen" is a reasonably common one. And while many people will not know its full scientific meaning, given that it's a scientific term that doesn't come up much in daily life, the companions are once again clueless to what it means, again done so the Doctor can explain its meaning in more detail to the audience. Especially egregious in Graham's case, who was not only married to a nurse, but also went through cancer treatments and the resulting immunosuppression, and bare minutes later "knows his way around an IV". Plus he's the oldest of the companions, and that alone should be enough to make him not knowing what it means unbelievable.
- Adam is left on the TARDIS with Jake while the Doctor's cure runs its course. The instruments on the TARDIS start beeping and glowing red while Adam appears to be getting worse. A bit later, Jake shows up with Adam, who is fine thanks to the cure.
- The aliens in Hazmat Suits turn out to be infected victims trying to find a cure, making it look like they're another Non-Malicious Monster. Then Suki reveals they infected the Earth so Praxeus would be Tested on Humans to see how it develops.
- Big Damn Kiss: Between Adam and Jake at the end, after the Doctor rescues Jake.
- Bizarre Alien Biology: The Doctor offhandedly suggests she has multiple brains.
- Body Horror: Creatures under the effects of an advanced Praxeus infection develop scale-like growths across their skin.
- Book Ends: The episode is bracketed by the Doctor talking about how all of the billions of humans on Earth are connected.
- Breather Episode: Another standalone episode with a mostly happy ending (the second in Series 12 after "Nikola Tesla's Night of Terror") kicks off the second half of a Story Arc-heavy season, right on the heels of the wringer that the Doctor was put through in "Fugitive of the Judoon".
- Brick Joke: Jake is trying to kick down a heavy metal door when Yaz and Graham arrive and unlock it with a skeleton key. Inside the building, Yaz and Graham find a flimsier wooden door which is also locked and try to figure out how to bypass it... until Jake lightly pushes Yaz aside and kicks it in.
- Continuity Nod:
- Like Clara in seasons eight and nine, Yaz has a go at being the Doctor, who sounds both impressed and annoyed at Yaz putting herself in danger.
- When asked what's the worst that could happen while being teleported to an unknown destination, Yaz replies You Do Not Want To Know, because the first time this happened she and the rest of Team TARDIS nearly died when they ended up in outer space.
- For the third time, the Doctor materializes the TARDIS around a ship about to explode, saving its occupant.
- This is not the first time the Doctor has been indicated to have multiple brains.
- Jake, like Brian Williams before him, is a man who dislikes travel but grows to appreciate it after meeting the Doctor.
- Creepy Crows: Praxeus is spreading itself across the planet by hijacking the bodies of birds that ingest plastic waste, resulting in murdersnote of infected crows flying around the polluted river in Peru and Suki's lab in Madagascar. Late in the episode, the Doctor and her companions are forced to flee when the infected crows attack the lab to try and stop her developing a cure.
- A Day in the Limelight: Yaz gets more time away from the Doctor and the team than usual, and has definite vibes of taking more risks.
- Did Not Think This Through: Jake tries to kick in a heavy, metal door. Which opens outwards. Good thing Yaz and Graham turned up when they did.
- Foreshadowing: Suki has a well-equipped lab in a remote corner of Madagascar, and when Yaz and Gabriela find that the building in Hong Kong is transmitting to two other locations, the island nation is one of them...
- Geeky Turn-On: After The Reveal that Suki Chang is an alien scientist, the Doctor kicks herself for being too distracted by her surprisingly well-equipped lab and ability to match her technobabble.
- Green Aesop: Gabriela takes Jamila to a river in Peru which she describes as the most beautiful river in the world, and finds its banks have become strewn with trash in the 3 years she was last there. Plastic pollution turns out to be central to the threat posed by Praxeus, and there are a few rants against humans introducing plastic into the food chain.
- Heroic Sacrifice: Subverted. Jake is willing to pull one of these by flying an alien shuttle into the atmosphere to spread the Praxeus cure across the planet, but the Doctor manages to save him moments before the shuttle explodes.
- Hoist by Her Own Petard: Suki immediately turns on the others once the Doctor develops a cure, stealing the data and transmitting it to the rest of her people. Unfortunately, she didn't realize that the cure was Earth-specific, which results in her death from a double-dose of Praxeus after she tries to heal herself with it.
- Homage Shot: Jodie Whittaker running on a beach towards an injured male person.
- Human Aliens: Suki belongs to an unnamed human-looking alien race that came to Earth to develop a cure for Praxeus. The Doctor, a Human Alien herself, doesn't realize she's alien until she is tipped off.
- In Medias Res: The episode starts with the Doctor and her companions already split up across the globe as part of their investigation in to the three distress calls they recieved in the previous episode.
- Insistent Terminology: Jake is very firm that he's not a former policeman, he's on a sabbatical. Yaz, who also is on sabbatical from her police job, is not impressed, as she doesn't claim to be a police officer while doing so.
- Interpol Special Agent: Jake claims he's in Hong Kong for Interpol, another sign that he's making stuff up.
- It Can Think: Suki warns the Doctor that Praxeus knows she's onto it. Sure enough, the crows attack in force moments later.
- It Makes Sense in Context: "We've literally got the most exciting rotting bird that I've ever seen!"
- Karmic Death: Suki is killed by the same virus she released on Earth.
- Malevolent Masked Men: The gas-masked, infected alien scientists who guard the Hong Kong lab.
- Next Sunday A.D.: The Doctor's narration places the story "early in the third decade of the 21st century", suggesting it's at most a year or two in the future from the broadcast date.
- No Biochemical Barriers: Played straight by Praxeus itself, which is equally as deadly to Earth lifeforms as it is to Suki's species, but averted by the cure the Doctor develops; the Doctor specifically created it to work on Earth lifeforms and when Suki tries to use the cure on herself, it only hastens the rate at which the Praxeus infection kills her. There's even a bit of Truth in Television in that the virus kills Suki and the other aliens more slowly as it evolved on their planet and so would be better adapted to their physiology, even if it's not adapted enough not to kill them.
- Noodle Incident: The Doctor twice mentions a talking cat in Ontario that she's ruled out as unconnected to the events of the episode.
- One-Word Title: "Praxeus".
- "Open!" Says Me: Jake seems fond of this. He's trying to bust down the door of the warehouse in Hong Kong when Yaz and Graham show up with skeleton keys and he actually does kick in the door of the room where Adam is.
- Patrick Stewart Speech: The Doctor's opening monologue, repeated in the closing, is about the interconnectedness of humanity.
- Pervy Patdown: Gabriela's pat-down of Ryan turns into this when she touches his chest and asks him if he works out.
- Plot-Demanded Manual Mode: The autopilot on the alien shuttle happens to break down moments after the Doctor notes that she's set it precisely so they won't be drowned by the ocean when the shuttle launches out of the garbage construct. Jake takes over so they can escape in the TARDIS and still stop Praxeus.
- Preserve Your Gays: Both Adam and Jake come very close to dying via Praxeus and Heroic Sacrifice, respectively, but are both saved in the end.
- Reduced to Dust: The human victims of Praxeus explode into dust as the infection kills them.
- Running Gag:
- Gabriela asking people if they recognize her from her vlog, Two Girls Roamin'.
- None of the companions know a specific piece of trivia critical to the task at hand.
- The Praxeus victims developing crusts on their skin and exploding into dust is similar to the time-delayed death of an apparent survivor of an alien attack in the final Quatermass serial. An astronaut returning to Earth infected by an alien virus is also similar to the first Quatermass serial.
- The crows attacking people is an obvious shout out to The Birds.
- An alien invasion is defeated by Earth's natural defences, though in this case enzymes rather than bacteria.
- Graham retorts to Jake, "Well, excuse me, Inspector Morse, but I ain't the fantasist round here."
- Space Whale Aesop: Humans need to learn to manage plastic waste responsibly to stop it entering the food chain... and because it could turn the planet into a haven for body-hijacking alien diseases.
- Straight Gay: Jake and Adam, enough so that it's difficult to tell exactly what their relationship is before Jake explains.
- Swapped Roles: Most of Yaz's scenes have her become the Doctor, with Gabriela as her companion.
- Understatement: Aramu's response to the TARDIS materializing in front of him is a surprisingly calm, "This is not an ordinary day." Which makes sense when it's revealed they are both alien scientists.
- The Virus: Praxeus is a pathogen with the ability to hijack the bodies of other lifeforms, spread through ingesting plastic or through injuries the infected cause to the healthy. According to Suki, it's also aware of any attempt to cure it and takes steps to sabotage the cure using its hosts.
- We Have the Keys: When Jake is trying to kick in a heavy metal door, Yaz and Graham turn up with a set of skeleton keys.
- Well-Intentioned Extremist: Suki is trying to save what remains of her species, and is willing to destroy humanity to do it.