The Doctor and K9 are about to go on holiday when a powerful force takes over the TARDIS. Summoned outside, the Doctor realises that only a guardian could have such power. The guardian informs the Doctor that he has been chosen for an important task: to retrieve the Key to Time, an artefact that maintains the equilibrium of time itself. Now and again there are times when the forces within the universe are out of balance and the Key is required to stop everything for a short time to enable the balance to be restored. Split into six segments, scattered throughout the cosmos and hidden, the Key needs to be reassembled for this purpose. The Doctor's task is to locate the six segments: he will be given a device which will take him to the location of each segment; the device will also reveal the segments, which are disguised. Additionally, the Doctor will be assigned an assistant, despite his protestations.

Back on board the TARDIS, the Doctor meets Romanadvoratrelundar - he chooses to shorten her name to Romana. She believes she has been assigned by the President and is excited to assist in the search for the Key to Time. Using the locator the TARDIS follows one of the segments to the planet of Ribos. On Ribos, Garron and Unstoffe arrange to secrete a rough diamond-like rock in a cabinet of relics: avoiding the shrivenzale beast which guards one entrance and the guards patrolling the main, public entrance. Later, Garron meets his highness, the Graff Vynda K with a view to selling him Ribos as a second home. There seems little to interest the Graff, despite Garron's pitch that the planet is very central and unspoilt. When Garron leaves to sort out passes for the Graff and his party, Graff discovers an old survey stating that the planet has a wealth of a valuable mineral - jethrik. If he can get his hands on a supply of jethrik, the Graff would be able to employ an army big enough to ensure that he wins back the Levithian crown.

Mr Fibula has prepared a report for the captain: all minerals have been mined, processed and stored; the captain is not pleased though - none of the more valuable minerals have been found. A new source has been located: the captain orders Mr Fibuli to make preparations to mine the new site. The captain then announces to the citizens to prepare themselves and watch for the omens and declares a new golden age of prosperity for all. One of the citizens, Pralix, listening to the announcment drifts away from a crowd, but he has become the focus of another group, the Mentiads, in hiding. They proclaim that a time of great evil approaches and they must prepare.

The continuing quest for the key to time leads the Doctor and Romana to Calufrax. However, as the TARDIS tries to materialise on Calufrax but something jams the materialisation. When the TARDIS finally does materialise instead of a boring, uninhabited icy planet they find they have arrived in a city on the world of Zanak. One happy citizen gives Romana diamonds and a ruby, telling her that the sky lit up, the omens have been good and the captain has promised riches for everyone. As he moves on, the citizen warns the Doctor and Romana to beware of the Mentiads. The sound of someone shouting out leads them to Pralix.

Pralix is tossing and turning, seemingly in a fever. His sister, Mula, their grandfather Balaton, fear that the Mentiads will come for him. Kimus, a friend is concerned that in spite of all the riches provided for them, the people have no real freedom. If the guards find out about Pralix they will kill him - and ? would allow them to do so, rather than let him be taken by the Mentiads.

The Mentiads are prepared and so they set out to harvest Pralix. The guards try to stop them but their weapons seem to have no effect...

A sacrificial ceremony is taking place in the middle of a stone circle on Earth. The worshippers of the Cailleach pour blood onto some of the stones, which glow and pulse in response.

The TARDIS materialises near to the stone circle some time later. Noticing strange tracks nearby, the Doctor and Romana continue their quest for the key to time. At the stone circle, they meet Professor Rumford and she assumes they are there to conduct another survey of the stones. There have been many surveys and over the years there have always been discrepancies concerning the number of stones. Together with Vivien Fay, the professor is carrying out a survey on the circle as well. Vivien remarks on the bloodstains the Doctor finds: a local druidic society make use of the stone circle for their ceremonies. They are led by Mr De Vries, an unpleasant man according to Rumford. deciding to visit De Vries, the Doctor sets off - Romana remains behind as her choice of footwear turns out to be unsuitable for the local terrain. The Doctor also feels that Romana should keep an eye on Rumford and Fay - there is something wrong, something he cannot put his finger on quite yet. Romana helps Rumford and Vivian with their survey. As the day comes to a close Romana chooses to remain at the circle, to wait for the Doctor to return. Rumford and Fay head back to the cottage where they are staying, nearby, extending an invite to Romana which she is free to take up later.

The Doctor has decided its time to take a day off. Romana reminds him that they are still searching for the segments of the key to time. The planet of Tara looks like a peaceful place: peaceful enough for the Doctor to indulge in some fishing while Romana locates the next segment. Romana agrees to seek out the segment, declaring that she will be back in one hour and then they can continue their quest. Sure enough, Romana finds the segment fairly quickly, but then some creature bursts out of the trees. She is saved from the beast's attack by Count Grendel. Apart from an injured ankle, Romana is all right and she thanks Grendel. He seems astonished by Romana's appearance and is also intrigued by the segment she is holding. Any such stone found on his land must be registered so he offers to take her to his castle to have her ankle attended to and the stone registered. Dsepite Romana's protests, Grendel insists that she come with him and then she can be on her way - after all, what difference could an hour make? Romana is surprised when she arrives at Grendel's castle and he orders her to be dismantled. Apparently he has assumed she is an android - and it appears she resembles an important person.

find the Doctor and warn him that he should not be fishing without permission. On learning that the Doctor knows something about electronics and machinery they take him to meet their prince. Prince Reynart has an android double which does not function correctly. Tomorrow he is due to be appointed king at the appointed hour, at the coronation room. Count Grendel and his guards stand in his way and if Reynart does not reach the room at the appointed time he forfeits his right to the crown. Grendel is also likely to try to assassinate Reynart - the android appears to be his one hope of living to become king.

Fenner has returned to from a meeting on Delta Magna, to the methane catalysing refinery on the planet's third moon. There are concerns that an environmental pressure group, calling itself The Sons of Earth, is attempting to smuggle weapons to the swampies, the natives of Delta Magna 3. A ship was tracked shadowing Thawn's as he returned to the moon: could it be a gun runner coming in under cover? There are stories of authorities losing track of Rohm-Dutt, a gun runner, in this region of space. If it is him, he will find the swamps dangerous and the men at the refinery have weapons of their own: to protect against attack, even from armed swampies. Searching for Rohm-Dutt, Thawn and Fenner spot the Doctor and conclude that he is the man they are watching for and shoot at him. Only on closer inspection do they realise that they have not found the gun runner. They take the Doctor back to the refinery as questions need answering.

Romana has been captured by the swampies: Rohm-Dutt is meeting with them, ready to supply the weapons they can use to drive the dryfoots off their world. Suspecting that Romana may have been sent to investigate his activities, Rohm-Dutt is content to leave her to whatever methods of torture the swampies have planned. The swampies decide to heed the old way of making a sacrifice to Kroll before battle...

The TARDIS materialises between the planets of Atrios and Zeos and finds itself in the middle of an interplanetary war, a war which Atrios appears to be losing. Princess Astra wonders how much longer this war can go on and asks the Marshal to plead for peace. The Marshal believes that peace can only be achieved by winning the war and that victory is near to hand. A strike on the hospital causes Astra to go and check on her people, to boost their morale. The Marshal knows that she also has feelings for surgeon Merak and allows her to go. Astra and Merak discuss attempting to contact the Zeons, but there has been no reply to their messages. There is nothing to suggest the messages are being blocked or intercepted - its as if Zeos wasn't there. Her escort reminds Astra of her other duties and takes her to meet some children. However, he locks her inside a room which is still irradiated from a previous strike. It appears a decision has been made to kill the princess.

The TARDIS has been detected and the Marshal suspects that it may be some Zeon device that allows the enemy to make successful strikes on Atrios. Destroying it could pave the way for a counterattack against Zeos and so the Marshal orders a missile strike. The Doctor removes the TARDIS from the path of the missile and materialises the ship on Atrios. Trying to locate a segment of the key to time, the Doctor and Romana arrive at the room in which Astra is trapped. The Marshal arrives and accuses them of killing Astra's escort and of being Zeon spies. With access to the TARDIS blocked, the Doctor and Romana must convince the Marshal they are not spies and provide a solution to the war...