When Graham (Bradley Walsh) wins an all-inclusive vacation for four through coffee loyalty coupons (?), he and The Doctor (Jodie Whittaker), Ryan (Tosin Cole) and Yas (Mandip Gill) are spontaneously teleported out of the TARDIS to a too-good-to-be-true spa called “Tranquility.” (Guess the TARDIS isn’t as impenetrable as it always seemed, if in the last episode a creature can push through its defenses in an attempt to board and this week its occupants can be snatched away effortlessly.)
Tranquility lives up to its name for about zero minutes. After being greeted by an affable alien named Hyph3n (Amy Booth-Steel), the Fam splits up to explore. Graham kicks back on a lounge chair, Yas meets vacationing seniors Vilma (Julia Elizabeth Fogle) and Benni (Col Farrell), Ryan contracts a machine-to-human virus (and is quickly sorted out by The Doctor) and The Doctor goes exploring.
As Ryan recuperates from his virus, and begins chatting up the fetching Bella (Gia Ré), alarms begin to sound for a “drill.”
Of course it isn’t a drill. The Doctor uses psychic paper to force Hyph3n to let her into the command center disguised as a linen cupboard, where she meets Kane (Laura Fraser), the owner of Tranquility. The Doctor quickly deduces that Tranquility is actually an off-world “fakecation” spot, an illusion within a climate-controlled dome. The truth of their environment is that Kane built the spa on an “orphan” planet, Orphan 55, which is uninhabitable due to nuclear fallout. And the drill is actually a native species, the Dregs, having breached the dome’s security and begun devouring the guests.
In no time, the resident numbers at the spa are diminished to those previously named, security guy Vorm (Will Austin), maintenance dude Nevi (James Buckley) and his son, Sylas (Lewin Lloyd, most prolific child actor in Britain? Recently of Judy & His Dark Materials). The Doctor gathers everyone in the “linen cupboard” until she can restore security and expel the Dregs, but once the threat has passed they discover that Benni has somehow gone outside the dome.
For some reason, literally everyone piles into a little truck to drive deep into Dreg territory in pursuit of the clearly doomed Benni. The truck, of course, gets disabled by a Dreg trap. Benni calls out from outside that he wants to marry Vilma, and that he also wants to be shot because he is being maimed.
In the ensuing race against dropping oxygen levels on their portable canisters and a swarm of violent Dregs, the dwindling group of survivors only wants to get back to the hotel and somehow to safety away from Tranquility. Everyone except Bella, as it turns out. She’s the one who brought the virus (and many bombs) because Kane is her mother, so absent that she hasn’t event recognized her estranged daughter. Bella wants to burn the whole resort to the ground to punish her mother for missing her birthday parties.
Another revelation as they navigate underground tunnels to get back to the hotel: They’re on Earth! A post-apocalyptic future Earth that succumbed to climate change-triggered nuclear war. How do they find this out? By seeing a sign in Russian indicating that the tunnels they’re running through are the abandoned Siberian subway system… and then by The Doctor confirming facts by reading the mind of a Dreg by touching its temple. Because she can do that. The Dregs are mutated human survivors of the apocalypse.
By the time they make it back to the hotel, pretty much all of the guest characters are dead. The Doctor helps genius Sylas repair the teleport, which allows him and his dad to get away and The Doctor and Fam to get away. Bella and Kane stay behind to heal their mother/daughter rift by trying to… defeat the Dregs? It’s unclear.
When they’re safely back on the TARDIS, The Doctor speechifies about how humans ignored science and let the planet become what they just saw– but that it’s only a possible future! It can be avoided by taking action NOW.
So that was fun.
Detail I skipped that I want to include here: Kane’s plan was to finance terraforming the Earth with her spa. This is real estate speculation, which The Doctor gets very judgey about. No mention later of the fact that even this ruined Earth is salvageable.