博士正在宇宙中飞行突然被截停，原来是白守卫出手干涉，他告诉博士宇宙中有数把时间之钥的碎片散播在时间和空间中，若不将他们收集而落到黑守卫的手中，宇宙将迎来空前绝世的灾难，热爱帮助的博士怎能错过这一切？回到塔迪斯之后博士和他的新同伴Romana开启了一场寻找时间之钥的冒险，而作为博士学妹的Romana在对博士傲慢和不理解慢慢转变成敬仰乃至携手唱平的无间搭档。

The ground-breaking soap set in a housing estate on the outskirts of Liverpool.

This season saw the introduction of the Black and White Guardians as well as the companion Romana, who became only the second female Time Lord companion since Susan. This season is sometimes colloquially known as The Key to Time season after the story arc involving the Key to Time. Although not the first season to consist of linked stories (Season 8 in 1971 was the first, as all the stories that year featured the Master), it was the first to be recognised as having a series long arc. Although multi serial arcs would occur again, such a full- season experiment wouldn't be attempted again until Season 23. The arc has a number of similarities to the Season 1 story The Keys of Marinus. The storyline was originally proposed for Season 15, but it required more planning.

The tracer detects the second segment on the planet Calufrax. The TARDIS makes a bumpy landing, and the Doctor and Romana soon discover that they are not on Calufrax at all. They are in fact on Zanak, a planet that has been hollowed out and fitted with engines so that it can transmat through space and materialise around others - such as Calufrax - to plunder their mineral wealth, leaving them as shrunken husks held by gravitational forces in a 'trophy room'. Ostensibly in charge is the Captain, whose body has been partially replaced with robotic parts following a near-fatal crash, but he is merely a puppet controlled by his nurse - a projection of Zanak's original ruler, the aged Queen Xanxia. Xanxia is using time dams, powered by the forces generated in the trophy room, to keep her body alive until this new, younger form has stabilised. The Captain prepares Zanak to 'jump' again - this time with Earth as its target. The Doctor attempts to thwart his plans, calling on the assistance of the Mentiads - a gestalt of telepaths amongst Zanak's indigenous population - to sabotage the engines. The Captain rebels against the nurse, but she kills him. The nurse herself is then destroyed by one of the Zanak natives whom the Doctor, Romana and K9 have befriended. The Doctor realises that the second segment comprises the whole of the planet Calufrax. He contrives to drop the compressed husks from the trophy room into a space-time vortex created by the TARDIS in the centre of Zanak, and is then able to pick up and convert the segment at his leisure.