Gastropods have taken over the planet of Jaconda, and they plan to cause an enormous explosion, in order to spread their eggs throughout the galaxy; kidnapping scientists from Earth in the process, in order to calculate the necessary mathematical equations. The newly-regenrated Doctor, and Peri, become involved with the planet's former ruler, elderly Professor Edgeworth, who is really the Time Lord Azmael. The two Time Lords are able to drive the gastropods from the planet, but Azmael loses his life in the process...
