UNIT is investigating a number of meteorite landings. The Brigadier has Cambridge scientist Liz Shaw seconded to UNIT to assist in the investigation. The main problem is that nothing was found at the site of the first landings. Now a cordon has been set up in the area of the newest landings. The Brigadier is pleased when he is informed that a man has been found unconscious lying next to a Police Box. His delight soon changes to disappointment when he visits the hospital where the man has been taken. Although the man recognises the Brigadier, as far as the Brigadier is concerned this is not the Doctor, a man who has helped him on two prior occasions.

After rumours spread that the mystery man might know something about the meteorites, someone attempts to kidnap him. The Brigadier increases the patrols and has the Police Box taken back to UNIT. The man at the hospital escapes and finds his way to the TARDIS. Could he be the Doctor, as he claims? If so, his help is needed in the investigation. It is not long before the investigation takes in Auto Plastics, a plastics factory which is pioneering a new process. Part of its work also involves supplying a new range of plastic mannequins for shop window displays and for museum replicas of well-known people - although they seem to be top ranking government officials rather than celebrities.

When one man, a former employee of the plastics company, appears telling a tale of a mannequin coming to life and trying to kill him it would seem that there is far more to Auto Plastics and the meteorites than meets the eye...

A series of caves in Derbyshire are playing host to a scientific project: a nuclear powered cyclotron is being used in important research - but there are problems hindering the research. Mysterious power losses, cave explorers dying or being rendered insane - when UNIT is called in to investigate, project director Dr Lawrence concludes that the Brigadier and his team are only a further hindrance. The arrival of the Doctor and Liz Shaw does nothing to ease his concerns. In Dr Lawrence's opinion it is all just teething problems and human error. Major Baker, the head of security at the base suspects saboteurs. The Doctor begins to conclude that something else lives down in those caves and it appears that Dr Quinn, one of the scientists at the base, may know more about what is down there than he is letting on.

As UNIT continues to investigate it becomes clear that Dr Quinn is trying to glean advanced scientific knowledge from the beings living in the caves. These beings, however, are trying to find out more about the human race - numbers, weapons etc - because Earth was once their world and now they want it back. Can the Doctor negotiate a peace between both species or are the more destructive factions on both sides going to win the day?

Mars Probe 1 has been en route back to Earth for 7 months - but with no contact from the astronauts on board. Recovery 7 has been sent up to dock with the probe and hopefully make contact with its crew. The eyes of the world watch as Recovery 7 docks with Mars Probe 1. The pilot continues to attempt to contact the crew of the Probe but when he hears nothing more he undogs the docking hatch and boards Mars Probe 1. Nothing more is heard from the pilot but a strange sound is heard. The Doctor identifies the sound as a message and asks the project director, Professfor Cornish, for access to a computer so that he can interpret the message and perhaps reply.

While they wait, however, a reply is sent: this second signal is traced to a local warehouse where the Brigadier and his troops encounter a small band of armed men. At first no one can determine what bearing all this has on Mars Probe 1, even when it appears to lead to an attempt to steal Recovery 7. It is only when Recovery 7 is finally opened and is found to be empty that UNIT must face the possibility that a hostile force has arrived on Earth...

Professor Stahlman is the director of a project to drill through the Earth's crust to a previously untapped energy reservoir of gas which he has named Stahlman's Gas. To thos involved in the project, the pace set by the director and the hear at the drill head have earned the project the nickname of Inferno. Executive Director, Sir Keith Gold, fails in his constant expressions of concern about the pace of the project - everything is being done with great haste. Even the experts that Sir Keith brings in are ignored by Stahlman: to him they are little more than obstacles, although one "expert" in particular gets under his skin - the Doctor. For the Doctor, the project also has the added bonus of allowing him to use some of the nuclear power of the reactor powering the drillhead. With this power he is able to conduct experiments with the TARDIS console - he still hopes to break free of the exile imposed upon him by the Time Lords. During one experiment, an energy surge causes the Doctor and the console to slip into another dimension. Luckily, Liz is able to reduce the power and the Doctor and the console return.

An investigation into cause of the power surge has a number of consequences: the discovery that some of the people who had previously disppeared are now turning up, but suffering some bizarre mutation; the discovery that these mutations are responsible for some of the deaths at the Inferno; and Stahlman becomes increasingly irritated at what he sees as constant interference in his project - and he reacts by shutting off the power to the Doctor's project. The Doctor, however, is able to sneak the power back on and when everyone is distracted he attempts another flight with the console. This time he slips fully through the strange dimension he entered before and finds himself on a parallel Earth...one where the Inferno is nearing completion...with terrifying consequences.