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.
While sightseeing in Paris, the Doctor and Romana find themselves repeatedly reliving the same moments in time. A visit to the Louvre Museum leads the Doctor to suspect an alien plot to steal the Mona Lisa.
Count Scarlioni has the Doctor, Romana and Duggan locked in his basement but the trio manage to escape and learn of the Count's experiments with time travel. Even more surprising are his multiple genuine copies of the Mona Lisa.
Travelling back in time to 1505 Florence in search of Leonardo da Vinci, the Doctor is surprised to instead find Count Scarlioni under a different name. Back in 1979, Romana and Duggan attempt to stop the Count's heist but arrive too late.
Count Scarlioni reveals his true form as Scaroth, the last of the Jagaroth and uses his time machine to go back in time and rewrite history. The Doctor, Romana and Duggan take to the Tardis in a race against time to stop him.
The Tardis lands on the overgrown jungle planet of Chloris where the Doctor examines an unusual giant eggshell but soon he and Romana are taken prisoner by the guards of Lady Adrasta, the planet's cruel ruler.
Descending into the pit, the Doctor meets the banished astrologer Organon who tells him of Lady Adrasta's monopoly over the planet's supply of metal. Back on the surface, Romana tries to prevent Adrasta from destroying K9.
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.
The Starliner Empress comes out of warp and collides with a scientific survey ship, the Hecate. The Doctor and Romana arrive offering to help separate the two ships but all is not as it seems.
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.
Discovering the Nimon power room puts the Doctor onto the immense threat facing the universe, all enabled by Soldeed's crazed plan to return his war-mongering people to military glory.
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.
