It is three years since the Sixth Doctor helped repel the Killoran invasion of Világ, and he brings a new friend, Mel, to meet an old one. Evelyn Smythe, his former companion is now married to Principal Triumvir Rossiter. But Evelyn has made political enemies, among them her own stepdaughter Sofia, and the reunion takes a deadly turn when Evelyn and Mel are kidnapped.
It is revealed in this story that Hex, who is a companion of the Seventh Doctor, is the son of Cassie from Project: Twilight and Project: Lazarus. The Doctor discovered this in The Harvest.