To break the centuries-old stalemate in their war, the Movellans need the Doctor, as do the Daleks Davros. The solution is simple, but neither side can see it, nor would the Doctor upset this perfect and unplanned peace they've achieved. Davros, though, is another matter.
As time slips a groove, the Doctor and Romana relive twenty-four seconds while on holiday in Paris. The second time it occurs the Doctor spots a woman using advanced, alien technology at the Louvre to study the security system guarding the Mona Lisa, but it's the Doctor who falls under suspicion of being a clever and masterful international art forger.
As the very well-mannered but callous Count Scarlioni rehearses his forthcoming heist of the Mona Lisa, the Doctor, Romana and Det. Duggan explore his basement, discovering a time travel laboratory and an entombed set of six inexplicably authentic copies of the Mona Lisa.
Pursuing the mystery of the multiple, authentic Mona Lisa's back to Leonardo de Vinci's workshop in 1505 Florence, the Doctor meets Captain Tancredi - the very same man he's met as Count Scarlioni in 1979 Paris. Politely but firmly invited to tarry, via a set of thumbscrews, the Doctor can't help but answer a few of the gracious Captain's most urgent questions.
Scaroth, the last of the Jagaroth, forces Romana to build a field interface stabilizer for his time machine. Romana reluctantly does so, not knowing that the full extent her action will lead to the total non-existence of all life on Earth. Humans will never have existed.
Answering a distress call, the TARDIS lands on Chloris, a planet choked with abundant vegetation. Discovering a gigantic, fragmented eggshell that's the source of the call, the Doctor and Romana also find themselves under arrest for tramping about in a zone marked for death. Before they can be brought before Lady Adrasta, the matriarchal ruler, for questioning and ceremonious execution, a roving ban of metal-scavenging bandits attack and run off with Romana.
With the Doctor down in the execution pit and presumed dead, the cruel Lady Adrasta threatens Romana in order to gain obedience from K-9 and use his blast ray in a hunting party against the globular alien creature lurking about in her mine tunnels, with additional designs on using the TARDIS to maintain her planetary monopoly on metal. Meanwhile the Doctor, alive, discovers Organon the Astrologer living down in the pit, having managed to avoid death by the strange, house-size alien creature that roams around down there, too.
Trapped face to featureless face with the alien creature, the Doctor tries his best to communicate with it. Meanwhile, as Adrasta's group tries to break through metallic bio-blockage created by the creature, bandits raid her palace for every scrap of metal they can find, including an alien shield that harbors powers no one suspects.
Full revelation of Adrasta's wicked acts in maintaining her planetary monopoly on metal brings her reign to an end, but the victimized alien creature holds a secret: massive interstellar retribution for her crimes is headed their way, with doom for all, including those whose current short-sighted actions undermine the Doctor's efforts to save the planet in order to become new leaders of the old metal monopoly.
A cruise ship loaded with passengers to the planet Azure collides with a trade ship as it comes out of warp, leaving the two ships merged but unstable. The Doctor, Romana and K-9 arrive to help out but discover there's also a problem with unidentified smugglers running a cargo of vraxoin, a forbidden organic drug that's as highly addictive as it is fatal. It instills the user with warm complacency and total apathy - the very qualities currently displayed by the cruise ship's bemused co-pilot.
The unstable interphase of the two ships opens up unexpected passageways for dangerous monsters caged in an electronic zoo aboard the cruise ship to escape. Meanwhile, as an attempt is made to hook Romana on vraxoin, the Doctor discovers a mystery man aboard ship to whom he gives chase.
Hiding out from arresting authorities in the electronic zoo's inhospitable Eden exhibit, the Doctor and Romana discover a man reported dead. Meanwhile, the monsters, called mandrels, start attacking and killing passengers, much to the amusement of the ship's captain.
After succeeding in separating the two merged ships, the Doctor discovers the identities of the drug runners. He, Romana and K-9 endeavor to catch them cleverly while also avoiding both the excise men wanting to shoot them and the marauding mandrels just wanting to kill anyone in sight.
After colliding with a spaceship, The Doctor, Romana and K-9 learn young natives from a peaceful planet called Aneth are being transported into a great labyrinth called "The Power Complex" where the evil bull-like Nimon demands that all his victims who enter The Power Complex are sacrificed so The Nimon can return to the Skonnan Empire back to glory. When Romana is taken captive and also chosen for sacrifice, The Doctor and K-9 travel to the heart of the maze as they set out to rescue Romana and foil The Nimon's evil plan to enslave the universe.
While the Doctor endeavors to amend putting Romana in great peril, she experiences a foretaste of Skonnos's fate on the ravaged planet Crinoth, and a shocked but undeterred Soldeed gets a glimmer of how he's been used by the Nimon.