The gang know that going anywhere with the Doctor will lead to adventure, so they try to take a different route with this one. They all go on holiday together to a place called Tranquillity, which is an idyllic, all-inclusive holiday destination where they can take some time out and relax.
But of course they get there and things go horribly wrong. They’re quickly dragged into a spiralling situation they’ve landed in the middle of.
The Doctor, Graham, Ryan and Yaz are taken to Tranquillity Spa via a transport cube for an all-inclusive stay. They are welcomed by their customer host, Hyph3n. As Ryan retrieves a snack from a vending machine, he is infected by a Hopper virus, which the Doctor is able to extract from him. While recovering, he meets a fellow guest, Bella. Meanwhile, a physical breach occurs and guests are asked to assemble for a muster drill. The Doctor convinces Hyph3n to give her access to the “linen cupboard”, which is actually a security room with an armoury, where they meet Kane. The Doctor becomes suspicious upon discovering that there is an ionic membrane needed to protect a holiday spa. The creatures that broke into the spa start killing guests. Making matters worse, the Hopper virus also found its way into the spa’s systems, disabling the transporter and security cameras.
The remaining survivors, the spa’s mechanic Nevi, his son Sylas, and an elderly woman, Vilma, meet at the “linen cupboard”, but Vilma notices Benni, her partner, has gone missing. The Doctor builds a new ionic membrane from scratch to banish the creatures. Now safe, Kane identifies the creatures as the local Dregs. It is revealed that the spa is a “fakation”, a place designed to look like a vacation but in a fabricated environment. The spa’s system, which tracks all the guests, shows Benni outside the spa, so the survivors go out to rescue him. From the vehicle, the group sees the uninhabitable desolation of the orphan planet they are on, Orphan 55. The vehicle is caught in a Dreg trap and the Dregs surround it, keeping Benni as a hostage. The group makes a run for a nearby service tunnel, but the Dregs kill Hyph3n and Benni is shot by Kane.
In the tunnel, Bella reveals she is Kane’s daughter, who the latter neglected in order to build the spa. Bella escapes with Ryan via the transporter while the others are forced to continue further along to the stairs as Dregs swarm into the tunnel. After the Doctor, Yaz, and Graham discover a rusted Russian sign, they deduce that Orphan 55 is actually Earth after it was devastated by global climate change and nuclear war in an unknown year. Vilma sacrifices herself to give the group more time to escape. Going through a Dreg nest, the Doctor learns that the Dregs are mutated humans that survived the fallout. Kane stays behind to give the group more time to escape. Bella resumes her plan to destroy the spa out of anger toward her mother. As the Dregs surround the spa to attack, the group fixes the transporter and safely evacuate, leaving Bella and Kane behind to fight off the Dregs. Back in the TARDIS, the group despairs over Earth’s future. The Doctor tells them that while this timeline is only a possibility, she cannot promise that it will not come to pass; humanity can either make a positive change or accept their fate and end up like the Dregs.