Reunited With The Brigadier

After the strange events encountered in the previous story (see "The Mind Robber") the TARDIS reforms itself and arrives in space on the dark side of the Moon. Without any warning The Doctor has to quickly relocate the TARDIS to avoid a missile attack. The TARDIS materialises in a field. However, on exiting the time machine The Doctor, Jamie McCrimmon and Zoe Heriot discover that the TARDIS's exterior has become invisible due to a damaged visual stabiliser circuit. They therefore set off to contact Professor Travers so as to get his help to repair the faulty circuit.

Unfortunately when they arrive at Professor Travers' home they find that he is away in America and his house is being rented by ex-model and photographer Isobel Watkins and her scientist uncle Professor Watkins. She suggests that her uncle may be able to help them and so gives them the address of International Electromatics's London head office where he is working. Zoe decides to stay and help Isobel take photographs leaving The Doctor and Jamie to visit Professor Watkins.

When they arrive a computerised receptionist won't allow them access, but when they try an alternative route into the building they are gassed and taken by the company's security chief, Packer, to see Tobias Vaughn - the Managing Director. He refuses to allow them to see Professor Watkins stating that he is at a delicate stage of his work and cannot be disturbed. After Jamie accidentally mentions the damaged part of the TARDIS The Doctor has no choice but to show Tobias Vaughn the damaged visual stabiliser circuit. Tobias Vaughn tells The Doctor that he will get Professor Watkins, when he has the time, to take a look at it. The Doctor though is suspicious of the Managing Director but has no choice but to part with this valuable piece of the TARDIS. On leaving International Electromatics The Doctor and Jamie do not get very far before they are abducted by two men who take them to a transporter plane at an airfield. Inside a big surprise awaits them when they are reunited with Alistair Lethbridge-Stewart, now promoted to Brigadier. The Doctor and Jamie learn that he is leading an organisation called the United Nations Intelligence Taskforce (UNIT) who are currently investigating International Electromatics.

The Brigadier

Meanwhile Zoe and Isobel become concerned when The Doctor and Jamie fail to return and so decide to go to International Electromatics in search of their friends. They too try to gain access via the computerised receptionist. Zoe though is determined not to be thwarted by the computer and ends up destroying it which results in the two girls being taken prisoner.

Back at Isobel’s house The Doctor and Jamie discover that Zoe and Isobel have followed them to International Electromatics and so return to the building themselves. At the rear they observe several large packing cases being loaded onto a train, one of which has an item of Zoe's clothing showing. But before they can investigate further the train departs and they are captured by Packer and his men. The Doctor is convinced that Zoe and Isobel have been kidnapped and so when they are brought before Tobias Vaughn again he, of course, denies The Doctor’s accusations but invites The Doctor and Jamie to travel to the company's compound in the country to see the train arriving.

Corporal Benton

At the compound there is no sign of Zoe or Isobel. The Doctor queries Tobias Vaughn about the deep space communicator he noticed when he came into the compound; in return, Tobias Vaughn demands that The Doctor explain about the damaged visual stabiliser, even threatening to hand Zoe and Isobel over to Packer if he doesn't co-operate. The Doctor and Jamie manage to escape onto a railway siding and hide inside a couple of crates on one of the trains. It is then that Jamie has a close encounter with a cocooned shape that begins to move.

The Doctor and Jamie are soon to discover that Tobias Vaughn is in league with the Cybermen who are planning to invade Earth. The Cybermen have been arriving in spaceships (so prompting a number of UFO sightings near the compound) and then being shipped in crates to London. There they are being hidden in the sewers of London until they are needed. The Cybermen also plan to send, from their spaceship that is hidden behind the Moon, a hypnotic signal through special circuits incorporated in all International Electromatics equipment. This signal will immobilise most of Earth's population allowing the Cybermen to carry out their invasion with little resistance. Tobias Vaughn is though plotting against the Cybermen with the aim of seizing power for himself. To achieve this he has been getting Professor Watkins to develop a machine that will generate emotional impulses, which can be used as a weapon against the Cybermen.

While rescuing Zoe and Isobel The Doctor learns of the Cybermen’s plans and tries to warn The Brigadier. Tobias Vaughn though has realised that UNIT is becoming dangerous to him and manages to stop UNIT's investigations. Jamie, Zoe and Isobel, on learning that The Brigadier is unable to investigate The Doctor’s suspicions, decide to venture into the sewers to find proof for The Brigadier but they narrowly escape from the Cybermen. Meanwhile Professor Watkins delivers his perfected machine to Tobias Vaughn and discovers that Tobias Vaughn has been partially Cybernised. Shortly afterwards UNIT mount a successful raid to free Professor Watkins. After The Doctor discovers the micro-electronic circuit, when he opens up an International Electromatics produced radio, he sets about making a device which will block the Cybermen's hypnotic signal. The Brigadier orders that this device is issued to all UNIT troops.

Tobias Vaughn and Packer

At dawn, on the planned day of the invasion, Tobias Vaughn activates a homing signal to guide the Cyberfleet to Earth while the Cybermen broadcast the hypnotic signal, causing the collapse of the human race; leaving the Cybermen to take over London. The only people unaffected are those wearing The Doctor’s device. As the Cybermen leave the sewers, and start to take over London, the UNIT troops start to attack them but they are hugely outnumbered and outgunned.

At the same time UNIT arranges for a Russian rocket to be launched, so as to destroy the source of the Cybermen’s hypnotic signal, while UK missiles are to be sent to destroy the incoming Cyberfleet which is homing in on Tobias Vaughn's transmitter. With help from Zoe the UK missiles are successfully launched and the incoming Cyberfleet is destroyed. However, the Cybermen blame Tobias Vaughn for the setback in their plans, and announce that they will now launch a megatron bomb at the planet. Furious at the Cybermen's actions Tobias Vaughn destroys the alien communication device in his office – so breaking all contact with the Cybermen. Realising that the Cybermen intend to destroy all life on Earth he agrees to assist The Doctor to stop the Cybermen. Using Professor Walkins' machine, and with the help of UNIT troops, they battle a massed army of Cybermen. In the battle Tobias Vaughn is killed but the UNIT forces succeed in shutting down the homing signal. Another missile destroys the Cybermen’s megatron bomb en-route to Earth and the Russian rocket destroys the last Cyberman spaceship so stopping the hypnotic signal.

With the crisis over, The Doctor, Jamie and Zoe return to the TARDIS where The Doctor installs the repaired visual stabiliser, turning the TARDIS visible again.