Picking up where Trail of the White Worm left off, the Master's allies invade the Earth. They capture the Doctor and take him back to their home planet for analysis, while they make plans with Colonel Spindleton to subjugate London, England and then the world. Meanwhile, people are not who they seem, and the Master's real plans proceed elsewhere...
- Call-Back: The villains and their methods are taken from "The Android Invasion". The Master refers to "The Deadly Assassin" and to the time that he tried to use the Eye of Harmony to restart his regeneration cycle.
- Cool Horse: Pink Gin, who's "not exactly Red Rum", but is a champion racehorse who Leela forms an instant rapport with.
- Easily Thwarted Alien Invasion: No one takes the Kraals seriously, and for good reason.
- Historical In-Joke: Pink Gin turns out to be Shergar.
- Our Wormholes Are Different: Rather uniquely created by the white worm of the previous story. Links Earth and Oseidon.
- Politically Incorrect Villain: Spindleton, whose rants about everything he thinks is wrong with England include workers' rights and women's lib.
- Robot Me: The Doctor, The Master (multiple versions!) and Tyngworg.
- Villain Team-Up: The Master and the Kraals. As with all teamups between the Master and an invading race, leads to Inevitable Mutual Betrayal. (In fact, by the end, pretty much every comibination of the Master, Spindleton and the Kraals have betrayed the others at least once. Well, if you count the Kraals and Spindleton betraying the Android Master, which may well have been part of the real Master's plan.)
- Weak-Willed: The Master accuses Spindleton of being the most weak-willed individual he's ever encountered. Spindleton says that the first Mrs. Spindleton said the same thing.