The Doctor decides to try Missy out on a test mission accompanied by Bill and Nardole, to a very long colony ship that's flown too close to a black hole. Missy is determined to have some fun, teasing her companions and mocking the Doctor in one of the series' most self-referencing scenes ever. However, the mission quickly turns serious when Bill is shot through the heart and mysterious figures carry her away.
Missy, earning the Doctor's trust, works together with him to try to solve the mystery of the ship and find Bill, but as she uncovers the ship's origins, her former self offers temptation that she finds hard to resist.
