The Doctor and Romana are enjoying a holiday in Paris, 1979, when they become aware of a fracture in time. During a visit to the Louvre to see da Vinci's Mona Lisa, the Doctor purloins from a stranger, Countess Scarlioni, a bracelet that is actually an alien scanner device.
He, Romana and a private detective named Duggan are then 'invited' to the chateau home of Count Scarlioni, where they find hidden in the cellar six additional Mona Lisas - all of them originals! The Count is revealed as an alien called Scaroth, last of the Jagaroth race. He was splintered in time when his ship exploded above primeval Earth, and in his twelve different aspects has since been guiding mankind's development to a point where time travel is possible.
His intention is to go back and prevent the destruction of his ship. To finance the final stages of this project, overseen by the misguided scientist Kerensky, he plans to steal the Mona Lisa from the Louvre and then secretly sell the multiple copies that one of his earlier splinters has forced da Vinci to paint.
The Doctor realises that the Count must be prevented from carrying out his plan as the explosion of the Jagaroth ship provided the energy that initiated life on Earth. Following Scaroth's trail in the TARDIS, he travels back to primeval Earth. Duggan fells Scaroth with a punch, thereby ensuring that history stays on its proper course.

The story introduces Lalla Ward as the newly-regenerated Romana. The TARDIS lands the Fourth Doctor and Romana on a strangely familiar planet. There, they meet the android Movellans, who are locked in a war with the Daleks. The planet is Skaro, and in a bunker sits an enemy long thought dead — Davros, creator of the Daleks.

Documentary about recording the cast album of the 1971 Broadway musical Company

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.

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.

This season introduced K9 to the series and saw the departure of Leela. It was originally intended to include Terrance Dicks' story The Vampire Mutations but, due to a major BBC production of Dracula, it was postponed. It later became Season 18's State of Decay. To replace it, Dicks quickly scripted the season opener, Horror of Fang Rock. The season took a two week transmission break over the 1977 Christmas period, between the broadcast of part four of The Sun Makers and part one of Underworld.