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.)

Ian and Barbara are accidentally taken into Smithers' laboratory and, while the Doctor and Susan face a hazardous journey to reach them, they witness the deadly effects of DN6 first hand.

The Doctor and his friends face a race against time to expose Forrester's plans and return to normal size before Barbara succumbs to the effects of DN6.

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.)

The Doctor and Ian are taken to the Dalek saucer while Susan and Barbara join the rebels in planning an attack.

The Doctor manages to escape from the Dalek saucer and meet up with Susan and David but Ian is left trapped onboard. Meanwhile, Barbara and her new friends face a dangerous journey across London.

While Susan and David try to find a way out of London, Ian and Larry infiltrate the Dalek mine workings.

The Doctor's party investigate the Daleks' central control in Bedfordshire while Barbara and Jenny's attempt to reach it are derailed when they are betrayed.

The Daleks' plan to transform Earth into a giant spaceship and wipe out its inhabitants is nearing completion and the Doctor and his friends face a race against time to prevent them from destroying the planet's core.

On the planet Dido, the monstrous Koquillion terrorizes Bennett and Vicki, the only survivors of a crashed spaceship. When the TARDIS lands, the Doctor, Ian, and Barbara team up with Vicki to discover what Koquillion is hiding. (Originally broadcast in two parts.)

The Doctor and Ian reach the crashed spaceship and meet up with Barbara and Vicki, where the Doctor uncovers the truth about Koquillion.

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.)

While the Doctor and Vicki meet with Nero, Barbara is taken to Rome to be sold while Ian is forced to work as a galley slave.

Ian and Delos are forced to fight each other for their freedom while the Doctor has to perform for Nero's court.

Ian and Delos manage to escape from Sevcheria and Tavius agrees to help Barbara rejoin them.The Doctor's attempts to avoid being thrown to the lions inadvertently result in Nero deciding to burn down Rome.

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 Doctor, Ian and Vicki are captured by the Zarbi while Barbara meets up with the butterfly-like Menoptera who are trying to reclaim the planet.

The Doctor is forced to co-operate with the Animus but Ian uses the opportunity to make an escape attempt.

Ian and Vrestin are captured by the subterranean Optera while Barbara and her Menoptera allies attempt to escape from the slave camp.

Barbara and the surviving Menoptera take refuge in the Temple of Light while the Doctor and Vicki attempt to escape by taking control of a Zarbi.

The Doctor and Vicki are taken before the Animus but the isoptope is left behind in the laboratory.The only hope of victory lies with Barbara and her Menoptera allies.

The TARDIS lands in the Holy Land during the Third Crusade and the crew help save Richard the Lionheart from an ambush, but Barbara is captured and brought before Saladin.

While the Doctor, Ian, and Vicki are drawn into the intrigues of King Richard's court, Barbara has to cope with an impatient Saladin and the vengeful El Akir.

Barbara allies herself with a man whose were murdered by El Akir as the Doctor and Vicki are caught up in Richard's plan to offer his sister's hand in marriage to Saladin's brother.

Ian sets out with a reluctant accomplice to rescue Barbara from El Akir's palace as the Doctor and Vicki try to evade the suspicions of the Earl of Leicester.