The TARDIS arrives on the planet Marinus on an island of glass surrounded by a sea of acid. The travellers are forced by the elderly Arbitan to retrieve four of the five operating keys to a machine called the Conscience of Marinus, of which he is the keeper. These have been hidden in different locations around the planet to prevent them falling into the hands of the evil Yartek and his Voord warriors, who plan to seize the machine and use its originally benevolent mind-influencing power for their own sinister purposes.
The Doctor, Ian, Barbara and Susan arrive in the TARDIS on board a spaceship. Their initial concern is for the ship's human crew, who are suffering from telepathic interference from the Sensorites, but Susan communicates with the Sensorites and finds the aliens fear an attack by the humans and are just defending themselves. Travelling to the Sense Sphere (the Sensorites' planet) the Doctor seeks to cure an illness to which the Sensorites and Ian have succumbed, but finds it has been caused by deliberate poisoning. The political manoeuvring of the Sensorite City Administrator poses another threat to the TARDIS crew as he seeks to discredit and implicate them.
A medieval English knight is magically transported to present day where he ends up falling for a high school science teacher.
A young filmmaker accidentally claps her idol’s mystical clapperboard, throwing the two on a frantic journey through film genres and beyond.
An 18th birthday mushroom trip brings free-spirited Elliott face-to-face with her wisecracking 39-year-old self. But when Elliott’s "old ass" starts handing out warnings about what her younger self should and shouldn't do, Elliott realizes she has to rethink everything about family, love, and what's becoming a transformative summer.
Mark Singer returns as Dar, the warrior who can talk to the beasts. Dar is forced to travel to earth to stop his evil brother from stealing an atomic bomb, and turning their native land from a desert into... well... a desert! Written by Jim Palin
On the planet Dido, the Doctor, Ian and Barbara meet two survivors from a crashed spaceship who are terrorised by the monster Koquillion.
The TARDIS crew take up residence near Rome in 64AD, where Ian and Barbara are kidnapped by slave traders, and the Doctor's imitation of Maximus Pettulian sees him taken to the court of Emperor Nero where he inadvertently plays a part in deciding the course of history...
On the planet Vortis, the Doctor and his companions aid the butterfly-like Menoptra in their battle against the parasitic Animus and its mind-controlled minions, the ant-like Zarbi.
In 12th century Palestine, the Doctor and his friends are drawn into the holy war between the forces of King Richard the Lionheart and the Saracen ruler Saladin.
An author, her reputation threatened by public humiliation, finds herself a prisoner, struggling to escape from a futuristic cult. Meanwhile, representatives of mankind debate with a time traveller to save the future of humanity.