The TARDIS materialises on a world: the locale is troubled with seismic activity; there is a high level of radiation; and it seems familiar in some way. Until the Doctor finds out more about the planet and why it seems familiar he decides they must stay and explore. As the Doctor and Romana explore they find the ruins of a city. The seismic activity appears to be the result of some massive underground excavation. Witnessing a surface burial, the Doctor investigates further and is intrigued to find that the body shows signs of exhaustion and malnutrition but not radiation poisoning. Attention is diverted by the sight of a ship coming in to land.

Before they can find out more about the ship a series of underground explosions drives the Doctor and Romana to seek shelter among the ruins of the city. The explosions cause part of a room to collapse and the Doctor is partially buried. Neither the Doctor nor Romana can free him so Romana must return to the TARDIS, complete the repairs to K9 and bring him along. En route to the TARDIS, Romana realises she is not alone. When the explosions cause the TARDIS to be buried Romana returns to the Doctor in the hope of finding another way to free him, omly to find that he has disappeared. Her mystery pursuer appears and Romana falls down a shaft into the underground of the city. There she encounters the Daleks...

One hundred and forty million years ago, a damaged ship is trying to leave Earth. Its pilot, Scaroth tries to advise his colleagues that using power level 3 to take off is dangerous but they override his concerns. The ship begins its take off and explodes...

Paris, 1979: the Doctor and Romana are enjoying the sights, sounds and the bouquet of modern France. Deciding that it is time for something to eat they visit a cafe. As an artist is drawing a picture of Romana, time skips. Apart from the Doctor and Romana, no-one else notices. After a brief discussion, the Doctor and Romana set off to visit the Louvre - to see the best painting in the universe: the Mona Lisa. Again, they experience a skip in time and the Doctor stumbles against some people.

Returning to the cafe, the Doctor and Romana examine a bracelet the Doctor pulled off the wrist of a woman who had also been looking at the Mona Lisa. The bracelet is not a product of current Earth technology, it is evidently being used to plot the theft of the painting. Why would an alien want to steal the Mona Lisa? One answer is provided by a detective, Duggan, who has followed them from the museum. He was observing the woman, Countess Scarlioni, as her husband has been selling off valuable paintings and artefacts, some believed lost for centuries. Its as if he has access to historical treasures and, if he could steal the Mona Lisa, there are seven potential buyers who would pay millions just to own it. Breaking into the Count's mansion, the Doctor, Romana and Duggan uncover a time travel experiment that could have dire consequences for all life on Earth...

Searching through the bits and pieces the Doctor has accumulated over the centuries, Romana finds a Mark 3 emergency transceiver. She plus it into the console and the device picks up some kind of signal. The TARDIS materialises on the jungle planet of Chloris, next to a masive wall-like structure. Investigating the wall, the Doctor and Romana are captured by Lady Adrasta's guards as they are in a restricted area known as The Place of Death. As they make their way through the jungle to Lady Adrasta's palace, a group of outlaws attack. They kidnap Romana and return her to their camp. Their leader, Torvin is not impressed - metal is a precious commodity on Chloris and there is little value in Romana, even if they were to ransom her.

Brought before Lady Adrasta the Doctor intrigues her with his theory that the wall is part of a shell and is the source of the signal the transceiver homed in on. If its crying out though, who is it cyring out to and why? Disappointed that her own people, Doran and Tollund, had not made such a conclusion, Adrasta decides that they must be punished. The Doctor watches as the unfortunate Doran is thrown down the pit, where dwells a mysterious creature. None who have been thrown to the creature have lived to tell more of it. Faced with being imprisoned by the Lady Adrasta the Doctor takes it upon himself to leap down into the pit...

The cruise liner Empress makes the jump to the planet Azure, but a lapse on the part of the 2nd officer, Secker, results in a collision with a survey ship, the Hecate just as it is about to depart from the planet. The captain transmits a mayday as the two ships collide, hyperspatially. Unstable interfaces are created as a result. The TARDIS materialises inside the Empress: the Doctor, Romana and K9 pose as members of a salvage company but propose to attempt to separate the ships rather than class them as salvage. Firstly, the Doctor needs to repair the power unit of the Empress. Secker is assigned to show the Doctor and K9 but partway there he gives them directions and heads off on his own. Curious, the Doctor and K9 follow him and discover that he is a vraxoin addict.

Waiting in the lounge with Dymond, Romana meets Professor Tryst and Della. Tryst proudly shows off his Continuous Event Transmutor (CET) machine: which stores small areas of planets onto an event crystal; flora and fauna from various worlds that continue to exist in stored form for continuous study. Romana is concerned that the unstable interfaces created by the collision might affect the dimensional matrix of the CET machine. Later on she checks out the machine herself: the Eden projection intrigues Romana as she is certain someone is watching her from within. Della enters the lounge and turns off the CET machine: Edden was the planet on which one member of Tryst's expedition was killed, someone that Della was close to.

Finding that the way to the power unit is blocked by an interface the Doctor must gain access by alternative means. Using K9 to cut through the bulkhead, the Doctor discovers that something else is able to travel through the interfaces. Something large, with claws and ready to kill...

Midway between Skonnos and Aneth, an old ship is returning to the former planet bearing a tribute cargo - comprising seven young people from Aneth and a quantity of hymetusite. The co-pilot is eager to return to Skonnos and hand the tribute over: then Nimon will fulfil his promise and provide the people of Skonnos with a new war fleet so that they can rebuild their once-great empire. The pilot remembers those glorious days, but memories alone are not enough for the co-pilot and he tries to override the computer - the sooner the ship returns to Skonnos the sooner the empire will rise again. His eagerness to reduce the remaining journey time from 12 to 6 hours does not pay off. Instead, the computer is overloaded, the automatic pilot cuts out, the ship drifts off course and the attempt to correct it damages the engines and the pilot is killed. With the ship adrift, the co-pilot tries to contact Skonnos for assistance.

The Doctor has immobilised the TARDIS with a view to trying out a new idea with the ship's conceptual geometer. He has dismnatled half the TARDIS systems, including the dematerialisation circuits. However, K9 reports that the TARDIS is not only moving, it is accelerating. Both the Doctor and Romana confirm this: some gravitational field is affecting the TARDIS. The Doctor had chosen a sector where there should are no black holes charted. Spotting a ship on the scanner, Romana is amazed that such a small object could exert such a gravitational pull - unless it too is caught in the same pull. The TARDIS collides with the ship and together they are being drawn towards the mysterious gravity field. Extending the door's defence shields, the Doctor and Romana are able to board the ship and offer to help repair it. Determining that the repairs will require using some of the precious hymetusite and parts from the TARDIS, the Doctor and Romana set to work, not knowing the co-pilot intends to use them to return to Skonnos and the promise of the 2nd Skonnan Empire.

A group of scientists are sitting, connected to a piece of apparatus. One of the group, Skagra, gets up and watches as the remainder twitch and finally lose consciousness. Activating a quarantine beacon Skagra carries off a sphere, leaving the re-awakening scientists to stumble around as if their minds have been drained. Skagra disembarks in a ship, from the Foundation for the Study of Advanced Sciences. His next stop is Earth, specifically Cambridge.

Chris Parsons visits Professor Chronotis at St Cedd's College, Cambridge - they had met at a college function and the professor has agreed to lending Parsons some books for his carbon-dating experiments. While the professor makes some tea, he informs Chris where he can pick up some books. Realising he is late for a seminar, Chris has to dash off - he will return the books the following week. In his laboratory, Chris notices that one book is written in some strange language. Attempts to analyse the book all fail, some equipment exploding.

The Doctor and Romana have also come to Cambridge, at the request of Professor Chronotis. A former Time Lord, Chronotis retired to Earth. He brought some Gallifreyan books with him, by accident, and now it is time to return them. One of the books is The Worshipful and Ancient Law of Gallifrey: such a book should never have been taken off the planet. Unable to find the book in the professor's collection, the time travellers urgently try to find it. Skagra is also searching Cambridge for the same book - which will give him access to the Time Lord prison planet of Shada...