The TARDIS is drawn by a mysterious force and lands on the alien planet Vortis. There the Doctor, Ian, Barbara and Vicki find themselves caught up in a war between an evil force known as "The Animus", its ant-like Zarbi allies, and the butterfly-like Menoptra race and their allies the subterranean Optera.
The travellers decide to take a holiday near ancient Rome, but after a month the Doctor gets restless. He and Vicki determine to visit Rome, and while they are gone Ian and Barbara are abducted by slave traders. All roads lead to Rome, however, and the travellers find themselves at the mercy of the country's petulant emperor, Nero... (Originally broadcast in four parts.)
Ian and Barbara believe the TARDIS has finally returned to their contemporary London, but soon discover signs of plague and warfare. Their old enemies, the Daleks, have invaded the Earth of the 22nd century. Separated and fighting for their lives, the travellers must try to aid the remaining humans and overthrow the Daleks' cruel tyranny. (Originally broadcast in six parts.)
When the TARDIS malfunctions and the doors open in the time vortex, the travellers and their Ship shrink to the size of insects. When they arrive on Earth, they uncover a murderous plot involving a new insecticide - but how can they stop the crime in their miniaturised condition ? (Originally broadcast in three parts.)