Zoe Menaced by the Cybermen

After the departure of Victoria Waterfield – who decided to remain on Earth – Jamie McCrimmon is upset despite The Doctor’s attempts to cheer him up. They land but The Doctor is unable to get the scanner to show what is outside the ship. The TARDIS it seems is trying to warn them that they have landed somewhere dangerous and that they should leave. However, before The Doctor can operate the controls the ship's fluid links overheat and mercury vapour floods the Console Room – forcing The Doctor and Jamie to make a hasty exit from the TARDIS. As they leave The Doctor manages to take with him the Time Vector Generator. The Doctor then informs Jamie that until the mercury can be replaced, the TARDIS is marooned.

They find themselves in a small spaceship which, after exploring, there is no signs of life or any mercury to refill the fluid link. While Jamie rests The Doctor continues to explore. Suddenly the spaceship changes course and The Doctor is thrown off balance and bangs his head. The unconscious Doctor is found by Jamie who helps him back to a crew restroom. But as they make their way there they are met by a servo robot. When the robot becomes aggressive, Jamie succeeds in destroying it using the Time Vector Generator. With the robot disabled Jamie realises that The Doctor’s condition is worsening.

Nearby, in a large space station known as The Wheel, the spaceship is picked up on their monitors and is identified as the Silver Carrier, a supply ship which had been missing for nine weeks and which is eighty million miles off-course. The station's controller, Jarvis Bennett, though is concerned that the drifting ship may prove to be a danger to The Wheel and, as it appears to be deserted, he orders its destruction using the station's x-ray laser. Luckily, just before the laser is fired, static is heard on the station's speakers emanating from the Silver Carrier – This is caused by Jamie using the Time Vector Generator to attract attention.

The Space Station

Jamie and the unconscious Doctor are rescued and brought onboard The Wheel. With The Doctor taken to the medical centre, Jamie is left having to explain their presence on the Silver Carrier - which he is unable to do. Jarvis Bennett is convinced that something is not right about the strangers and so The Wheel's librarian Zoe Heriot is tasked with showing Jamie around the station and to observe him discreetly. As the drifting Silver Carrier still poses a risk to The Wheel Jarvis Bennett orders its destruction. When Jamie learns of this order he sabotages the laser, by spraying liquid plastic into the device, so as to prevent the destruction of the TARDIS which is still aboard the Silver Carrier.

Un-noticed by anyone several small white bubbles emerge from the Silver Carrier and float across space before attaching themselves to the surface of The Wheel. Also in the hold of the Silver Carrier two larger versions of the bubbles start to move. Then without warning a metallic fist suddenly bursts through the outer membrane and a Cyberman emerges…

The Servo Robot

With the now-active Cybermen monitoring The Wheel they activate Cybermats which had been sent to The Wheel in the small white bubbles. There they destroy The Wheel's supply of Bernalium, a metal vital to the function of the laser. One of the Cybermats though is caught when a technician is attacked, and killed by one of the Cybermats, but not before he manages to cover it with liquid plastic. Zoe and the recovered Doctor use an x-ray machine to see inside the hardened plastic and it is then that The Doctor realises that the Cybermen must be aboard the Silver Carrier.

The Doctor tries to warn Jarvis Bennett about the Cybermen and his theory that the Silver Carrier did not drift to this sector but was deliberately piloted there. The controller though does not listen. He is more concerned with an approaching meteorite shower heading straight for The Wheel, and the fact that the laser is still out of action. Desperate for Bernalium he orders scans to be made to locate some. None is found on The Wheel but the scan picks up a strong presence of Bernalium on the Silver Carrier. Two technicians are therefore sent over to the Silver Carrier to retrieve it. However, it turns out that this is a trap set by the Cybermen – who take the technicians prisoner. The Cybermen then hypnotise them into smuggling the Cybermen onto The Wheel inside the crates of Bernalium.

The Doctor realises that the Cybermen must have hypnotised the technicians, and that they must have an ulterior motive for invading The Wheel. The Doctor is right the Cybermen need The Wheel to provide a radio beacon to direct a Cyber-invasion fleet on its way to attack Earth. To prevent more of The Wheel’s crew from becoming hypnotised he asks them to construct devices to block the Cybermen's control signals. Soon after and with most of the crew wearing the metal signal blockers fitted to the back of their necks – and so protected from the influence of the Cybermen - they are able, with the help of The Doctor, to set a trap for the Cybermen. When The Wheel is back in the human’s control The Doctor repairs the laser and enhances its power with the Time Vector Generator. It is then used to destroy the Cybermen's ship.

The Doctor and Jamie

But they are too late to prevent a troop of space-walking Cybermen to leave their ship, before it is destroyed and head towards The Wheel. Jamie manages to kill the remaining Cyberman on The Wheel, by spraying it with liquid plastic, and then, with the help of a crewmember, repel the approaching space-walking Cybermen using a Neutron Force Field.

With the Cybermen defeated and The Wheel safely back in the control of the humans, The Doctor and Jamie return to the Silver Carrier with the mercury they need to repair the TARDIS. Before they depart they discover Zoe hiding in the TARDIS. When she is discovered she asks to join them. But before he agrees to Zoe’s request The Doctor decides that she should know something about the dangers she may face. The Doctor uses a thought visualiser, to project images from his mind to the viewscreen, of his last encounter with the Daleks.