In the ocean on an alien world, there's a city run by scientists. The Doctor is wants to help them find new ways to produce clean energy. But others have recognized the value of the sea base, and their interest is not so benign.
When the TARDIS is trapped in the Vortex, the Doctor and Adric realize that there is something immensely powerful that is draining time itself from the universe.
Bill Potts has an unexpected visitor: another Bill Potts from the future. Bill summons the Doctor, who confirms that the other Bill is genuine. She has come to see her younger self for a reason - but she can't say what it is.
At last, the TARDIS has got it right. The Doctor, Adric, Nyssa and Tegan have landed in London, 1982. And yet - The Palace is home not to a Queen but to an Emperor.
When the TARDIS crashes to earth before his eyes, the Fourth Doctor is reunited with his former self and another Sarah Jane. But are the four of them enough to keep the Sand Box closed and whatever is within from getting out?
The Twelfth Doctor embarks on a desperate search for the three scattered parts of the Triskelia. But two warring races, the Myzen and the Tollatee are also searching for this powerful device, as is a mysterious mercenary.