In the UNIT Lab

A weather balloon carrying a device for cosmic ray research comes down at a wildlife sanctuary. It is found by the local gamekeeper, Mr Ollis but he suddenly vanishes when the weather balloon’s owner, Doctor Tyler, arrives to collect his device. Doctor Tyler takes the device to UNIT HQ as previous results of his tests have shown an unknown type of cosmic lightning, directed at the Earth. The Doctor is intrigued and goes to see the area where the balloon came down.

While The Doctor is gone, Doctor Tyler develops the latest negative from his device and is shocked to discover that it shows an image of Mr Ollis. Then without warning Doctor Tyler also suddenly vanishes and a crackling organism emerges from the box and goes down a nearby sink plug hole. Shortly afterwards The Doctor and Jo Grant arrive back at UNIT HQ they are confronted by the organism, when it emerges from a drain, and Bessie vanishes. The Doctor deduces that the creature is hunting him and that they need only wait before it tries again.

It is not long before, UNIT HQ is besieged by strange orange creatures, known as Gell Guards, that suddenly appear out of thin air. A full scale battle is soon in progress forcing The Brigadier to order the evacuation of the building. The Doctor, Jo and Sergeant Benton, however, become trapped inside, by the organism, forcing The Doctor to usher Jo and Sergeant Benton into the TARDIS so as to escape. The Doctor though is still unable to get his TARDIS to operate and so is forced into sending a message to the Time Lords to request their assistance. But the Time Lords have problems of their own as all their energy is being drained away through a black hole. The Chancellor informs the President of the Council that they are up against a force in a universe of anti-matter. The only way to assist The Doctor is to call on the help of his previous incarnations even though this will break The First Law of Time which expressly forbids any Time Lord to cross his own time stream and meet his former (or future) selves.

The Gell Guards

Meanwhile back on Earth, in the TARDIS, The Doctor discovers a recorder on the TARDIS console which is suddenly claimed by his previous self who appears from nowhere. After explaining who this stranger is the Second Doctor and the Third Doctor decide to link minds so that they learn from each other what has happened so far. However, both incarnations start to argue about the best course of action and so, to stop them arguing, the Time Lords send the earliest incarnation of The Doctor. He appears on the TARDIS' scanner screen and points out to his other selves that the entity outside is a bridge.

This revelation prompts the Third Doctor to make a decision. Without warning he switches off the force field and exits the TARDIS, closely followed by Jo. As soon as they leave the safety of the TARDIS they both vanish and the organism calms down. The Brigadier then returns and is astonished to see the Second Doctor, instead of the Third Doctor. Despite not understanding what has happened he orders the Second Doctor to find a way of subduing the organism.

Sergeant Benton

On a bleak and rocky planet the Third Doctor and Jo start to explore their new surroundings. They discover parts of UNIT HQ and even Bessie have been transported to this alien world. They also discover Doctor Tyler. However, unknown to them they are being watched and so all three are soon captured by more of the Gell Guards and escorted to a large palace. The Doctor soon discovers its owner is Omega, a hero from Time Lord mythology who was a solar engineer who arranged the detonation of a star in order to find and create a power source to give the Time Lords mastery over time itself. Omega claims he was sacrificed to the resultant super nova and now wants revenge over the Time Lords and so he created this anti-matter world by the force of his will. But now he has become trapped in this universe, unable to leave. His dilemma is that in order to escape he must abandon control and if he abandons control then he cannot escape.

The Second Doctor meanwhile, under The Brigadiers orders, has rigged up a device to pacify the organism that is still inside UNIT HQ. But when Sergeant Benton uses it, it enrages it instead forcing the Second Doctor, Sergeant Benton and The Brigadier into the TARDIS to escape. The First Doctor then appears on the screen once more and advocates turning off the TARDIS’s force field. The Second Doctor complies and the whole of UNIT HQ is transported through the black hole.

On exiting the TARDIS, The Brigadier is convinced they have landed at Cromer and so goes off to summon help. Unable to stop him The Second Doctor, with Sergeant Benton, sets off to explore but they are soon captured by the Gell Guards and find themselves imprisoned in a cell, along with the others, in Omega’s Palace. The Second Doctor and the Third Doctor though are able to match their wills against Omega and create a door in the cell so enabling them all to escape. Both incarnations of The Doctor head for the singularity chamber while the others escape from the palace where they meet up with The Brigadier who has also found Mr Ollis.

The Second and Third Doctors

In the singularity chamber Omega soon discovers The Doctors. It is then that they learn that Omega wants The Doctor to take his place so that he can then escape. But they also discover that all that remains of Omega is his will – and so he can never leave. Omega is infuriated by this revelation and in his rage The Doctors manage to escape from the palace and reach, along with the others, the safety of the TARDIS.

The First Doctor then appears on the screen again and between all three of them they realise that they can use the TARDIS's force field generator to destroy Omega. However, when they go to disconnect it, the find that the Second Doctor's recorder has somehow become lodged inside. This gives the Third Doctor an idea. They trick Omega into allowing them to move the TARDIS to his palace where they then persuade him to let all the others return to Earth. Once everyone has returned home the Second Doctor and the Third Doctor then offer Omega the TARDIS's force field generator with the Second Doctor’s recorder still inside. Omega though sweeps it from their hands in disgust and there is a massive outpouring of power, turning the black hole into a super nova, returning The Doctors and the TARDIS to UNIT HQ on Earth and restoring power to the Time Lords.

The Third Doctor then explains that the recorder, being stuck inside the TARDIS's force field generator, had not been converted to anti-matter, and so when Omega knocked it from the generator, the collision of matter and anti-matter destroyed his universe, snapping everything else back to its rightful place. As the Third Doctor, Jo, The Brigadier and Sergeant Benton say their goodbyes to the First and Second Doctors the Third is upset at having been forced into tricking Omega. However, his regret is soon forgotten when a new dematerialisation circuit appears on the top of the TARDIS's central column, and The Doctor's time travel knowledge and codes come back to him. The Time Lords have forgiven him and have restored his freedom to travel in time and space once more…