The Brigadier (Nicholas Courtney) is suffering a visit by Chinn (Peter Bathurst), head of a committee of inquiry, who is concerned that there seem to be no files or records on the Doctor. Bill Filer (Paul Grist) from Washington is also in England to help UNIT track down the Master. While the Brigadier, the Doctor and Jo are meeting with Chinn and Filer, two UNIT radar operators (Michael Walker, David G. March) detect a spacecraft of variable mass approaching the Earth. Chinn assumes command and orders a missile strike on it, but the object vanishes. The missiles are aborted and the object lands on the South East coast of Britain, close to the Nuton Power Complex which supplies Britain with power. Filer is dismissed by Chinn and UNIT mobilises to go to the landing site. 

The ship's landing is seen by a tramp, Pigbin Josh (Derek Ware), who is captured, analysed and rejected. Bill Filer arrives just before UNIT and he too is captured by the ship. It determines to hold him for further analysis. Filer discovers that the Master (Roger Delgado) is also prisoner on the ship 

Setting up a mobile HQ, UNIT hear a heartbeat sound and a message asking for help coming from the ship. The Brigadier, the Doctor and Chinn, together with Sir George Hardiman (Donald Hewlett) and Doctor Winser (David Savile) from the Nuton Complex, go in. Inside, they meet the occupants, beautiful golden humanoids called Axons (Bernard Holley, Patricia Gordino, John Hicks, Debbie Lee London). They wish to remain on Earth to replenish their energy and in return they offer Axonite, an intelligent substance that can be made to mimic any other element. The Doctor is immediately suspicious. If the Axons have access to such a wonderful substance, then why did they run out of fuel? 

The humans take a sample of Axonite away for study, but Chinn - who aims to keep Axonite for exclusive use by Britain - gains special powers from the Minister (Kenneth Benda) and has the regular army place all the UNIT personnel under guard. The Doctor and Winser take the Axonite to the Nuton Complex to start analysing it. The Doctor wants to put the Axonite in the particle accelerator and get it to analyse itself, but Winser won't agree to his equipment being used in this manner. The Doctor also thinks that he might be able to get the TARDIS operational again with the help of the equipment. Winser leaves to arrange for the transport of the TARDIS to the Complex. 

Meanwhile the Master is allowed to leave Axos to ensure that the Axonite is distributed globally - thus allowing Axos to drain the Earth of energy. The Master brought Axos to Earth in exchange for his life and the destruction of the Doctor and all life on Earth. Axos holds his time capsule to ensure his cooperation. Axos creates a duplicate of Filer to bring the Doctor back to them but the real Filer escapes. The Master hitches a lift to UNIT HQ on the lorry sent to collect the TARDIS. There, he arranges for a message about Axonite to be sent to the UN, thus ensuring that the other countries will ask for some of the substance for themselves. He then returns to the Complex with the TARDIS. 

The fake Filer and the real Filer fight. The fake Filer falls into the particle accelerator and is destroyed. The Doctor and Filer report to the Brigadier and Filer is placed under guard with the rest of UNIT. With Winser away, the Doctor places the Axonite in the particle accelerator. He realises from the results that Axos, Axonite and the Axons are all just different aspects of a single organism. When the speed of light is reached in the accelerator, the substance starts reacting and absorbing all the energy. Axos panics as its nutrition cycle has been started early. It orders the retrieval of the Axonite. 

At the Complex, the UNIT troops break out from confinement and Filer and Jo rush for the lab. Winser arrives and tries to open the particle accelerator door. He is transformed into a rolling glob (Douglas Roe, Clive Rogers, Eden Fox, Stuart Myers) of Axon material. Axon monsters invade the lab, stun Filer, retrieve the Axonite and take the Doctor and Jo back to Axos. 

The Brigadier assumes command but Chinn is ordered by the Minister to arrange for world-wide distribution of Axonite immediately. 

By threatening Jo with rapid aging Axos gets the Doctor to reveal the theory of time travel. They realise that they need power from the Nuton Complex to achieve it. The Axon man is ordered to enter the reactor and the resultant power build-up is steadily absorbed by Axos. 

The Master arrives at the complex with the TARDIS and starts to effect repairs to the Doctor's ship. He is seen by Yates (Richard Franklin) and placed under arrest. However the reactor is overloading, and the Brigadier has no choice but to allow the Master to help. The Time Lord uses the TARDIS to store up the power, and then releases it all back at Axos in one go causing the ship to convulse. This allows the Doctor and Jo to escape. The power surge is still continuing, however, and Hardiman orders the complex evacuated. He tries to disconnect the cables and is killed. 

The Doctor does not know how to stop Axos and so asks the Master for help. He says that he wants to leave Earth to Axos and the Master agrees to help by reparing the TARDIS while the Doctor calculates some course coordinates. 

Axos starts its nutrition cycle and the Doctor and the Master leave in the TARDIS, but their destination is Axos. The Doctor offers to link Axos and the TARDIS together to give them the power of time travel on condition that they join forces against the High Council of Time Lords. The Doctor links the ships, but throws them into a time loop. The Master escapes in his time capsule while Axos is distracted. 

At the moment Axos enters the time loop, the Axons attacking the lab vanish and the Brigadier orders everyone out before the place blows up. The Doctor manages to break the TARDIS free of the time loop, leaving Axos trapped forever. The TARDIS arrives just before the lab blows up and the Doctor hastily leaves again. He finally returns after the explosion has happened. He later admits that the TARDIS has been programmed always to return to Earth and that he cannot escape.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}