The TARDIS arrives in Killingworth in 19th Century England, where the Master is plotting to kill some of the key figures of the industrial revolution. Also present is the Rani, a Time Lord biochemist who was cast out from Gallifrey and is now dictator of the planet Miasimia Goria.

The Rani has altered the metabolism of Miasimia Goria's populace to heighten their awareness, but in the process has inadvertently lowered their ability to sleep. In order to correct this she has been drawing a fluid from the brains of humans at various points in Earth's history, unconcerned that this leaves them aggressive and unable to sleep themselves.

Now she is adding to the unrest caused in England by the Luddites. She has also planted some land mines that turn people into trees.

The Doctor sabotages the Rani's TARDIS and she and the Master are sent spinning into the vortex at the mercy of a rapidly growing tyrannosaurus rex embryo - a specimen collected on one of her earlier visits to Earth.