The Twelfth Doctor embarks on a desperate search for the three scattered parts of the Triskelia - the only means to contain a dangerous multi-dimensional breach in reality: the Chaos Cascade. But two warring races, the aggressive Myzen and the peace-loving Tollatee are also searching for this powerful device, as is a mysterious tech-enhanced part-human mercenary.

Meanwhile, the Doctor's former housemate Tania Bell travels to Yorkshire to investigate reports of alien activity and is delighted to run into a previously unknown incarnation of the Time Lord: one that is female, dresses in purple and carries a parasol!

Can the Doctor, Missy and Tania find a way to work together as a team? Or, once unleashed, will the Chaos Cascade wreak havoc across all space and time?