In orbit around the planet Azure in the year 2116, a luxury space liner, the Empress, and a private ship, the Hecate, collide as the liner emerges from hyperspace and the ships become fused together. The Doctor and Romana arrive to investigate.

The travellers meet the scientist Tryst who has a Continuous Event Transmuter (CET) machine with him. On this are recordings of many of the planets that he has visited, which continue to develop within the machine. The Doctor discovers that someone on the liner is smuggling the dangerous addictive drug Vraxoin and that some Mandrels from the mud-swamps of Eden have somehow escaped from the CET machine and are running amok. 

The culprits are revealed to be Tryst and the pilot of the Hecate, Dymond, who are planning to beam the Vraxoin down to Azure, the planet below, by sending the sample of Eden from the CET machine.

Vraxoin turns out to be the material that the Mandrells decompose into when they are killed. It is also discovered that the CET machine does not take recordings of planets, but has actually displaced whole planetary areas into its recording crystals.

The Doctor prevents Tryst and Dymond from carrying out their plan, separates the two ships and returns the Mandrels to Eden.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}