A marooned alien spaceship, piloted by a green one-eyed creature, tries to take off from primeval Earth but explodes.
In a chateau in modern-day Paris, a clumsy scientist Professor Kerensky complains of lack of funds for his experiments. His employer, Count Carlos Scarlioni, grants him a million francs, procured from the sale of rare artifacts and works of art.
The Doctor and Romana, holidaying in Paris, become aware of brief distortions in time. During one such incident near the Mona Lisa at the Louvre Museum, the Doctor steals a woman's bracelet, which he subsequently identifies as a "micromeson scanner", used to monitor the Louvre alarms. Duggan, a detective, marches the Doctor and Romana into a café at gunpoint. He explains he is investigating the sudden appearance on the market of masterpieces thought lost for centuries. Thugs soon capture the trio and take them to the chateau of Count Scarlioni, whose wife was wearing the bracelet. Unsatisfied with their answers, the Count has them locked in a cell in the basement where Kerensky is carrying out his time manipulation experiments. The Doctor warns the Professor of the dangers and explains that rather than manipulating their own space-time continuum, Kerensky has created a separate, incompatible pocket universe.
Duggan breaks through a brick wall to a hidden room and finds six Mona Lisas, all genuine according to the Doctor's expert eye. Duggan knows of seven people who would be willing to pay for the Mona Lisa for their private collection, but not if it were still hanging in the Louvre; hence the Count's plan to steal it.
The Doctor returns to the TARDIS and takes it to Florence, Italy in 1505 to see Leonardo Da Vinci. There, a guard tells the Doctor that Da Vinci is engaged on important work for Captain Tancredi. Count Scarlioni's exact double, Tancredi identifies himself to the Doctor as Scaroth, the last of the Jagaroth. The Doctor has heard of the Jagaroth: they destroyed themselves in a war 400 million years ago. Tencredi explains that a few escaped to the young planet Earth in a dilapidated spacecraft. The ship disintegrated upon takeoff and Scaroth was fractured, with splinters of himself scattered across time and space. As Tancredi becomes distracted while communicating with his other selves, the Doctor scrawls 'THIS IS A FAKE' on the blank Mona Lisa canvases and leaves a note to Da Vinci to paint over it. He then escapes back to modern-day Paris.
Kerensky learns the real reason for his time experiments: to reunite the various fractions of Scaroth and to get him to the spaceship before it explodes. Kerensky becomes unco-operative and the Count forces Romana to help. Once she agrees, he kills Kerensky.
The Doctor tells the Countess who her husband really is. Eventually she understands and confronts the Count, who reveals his true face: a green tentacled one-eyed creature - Scaroth. He activates her bracelet, which kills her.
Meanwhile the Doctor explains to Romana the enormity of Scaroth's plans. He tried to put the whole Earth into the time bubble by 400 million years, but only succeeded for a few seconds, which caused the earlier time slips. Now that he has the device she constructed, the field will be stabilised. However, Romana reveals that she has rigged it so that he will only be in the past for two minutes.
The Time Lords and Duggan escape the lab, only to be confronted by the monstrous Scaroth, training a gun upon them. He knows of the limitations Romana has placed upon his machine, but believes it makes no difference; he will still be able to prevent his spaceship from exploding, thus saving his race and remaining unsplintered. He vanishes, and the machine explodes. The trio dash through Paris to retrieve the TARDIS.
The TARDIS arrives on a barren plain, and they soon spot the Jagaroth spaceship. Romana observes that the ship's thrust motors are disabled and that the aliens will try to take off on warp drive. The Doctor points out the primordial slime from which all life on Earth will develop, currently an inert slurry. The explosion which annihilated the Jagaroth and caused Scaroth to splinter also caused the birth of all life on Earth.
Scaroth appears and makes his way to the ship. The Doctor tells him that he can't change history, but Scaroth won't listen. Duggan punches Scaroth and knocks him out. Scaroth's two minutes are up and he vanishes. The others rush back into the TARDIS. The Jagaroth ship explodes, thus creating life on Earth.
Back at the chateau, the Count's butler watches as a monstrous, one-eyed creature appears in the time field. Not realising the creature is his master, he hurls an object at the machinery, destroying the equipment, setting the chateau on fire and blasting the Count to oblivion.
The Doctor and Romana return Duggan to Paris and bid goodbye.
Doctor Tom Baker (Fourth Doctor)
Companion Lalla Ward (Romana II)
Julian Glover — Count
Catherine Schell — Countess
David Graham — Kerensky
Kevin Flood — Hermann
Tom Chadbon — Duggan
Peter Halliday — Soldier
Eleanor Bron, John Cleese — Art Gallery Visitors
Pamela Stirling — Louvre Guide
Writer "David Agnew" (David Fisher, Douglas Adams and Graham Williams)
Director Michael Hayes
Script editor Douglas Adams
Producer Graham Williams
Executive producer(s) None
Production code 5H
Series Season 17
Length 4 episodes, 25 minutes each
Originally broadcast September 29–October 20, 1979