The TARDIS arrives in the nick of time. It lands just outside the Dome and the Second Doctor, Jamie McCrimmon and Victoria Waterfield soon get inside and into the control area, where the Doctor offers his services to help operate the Ioniser, announcing that in 2 minutes, 38 seconds, the Ioniser will explode. His nimble action saves the Ioniser and coupled with his scientific analysis of the current ice age – that it has been caused by a severe drop in the carbon dioxide level of the atmosphere following the wholesale extermination of plant life – Clent is persuaded of his usefulness, despite initial misgivings. The Ioniser is being used to reflect and increase the sun's rays onto the ice in order to melt it, but the process is highly complex and Britannicus Ice Base in particular is failing to stop the ice flows.
Arden and Walters have now reached the base with their prize, the nature of which takes the crew by surprise. Arden sets up an electrical device designed to melt the ice around the warrior. The Doctor examines the frozen man and concludes he conforms to no known civilisation. As the creature’s helmet seems to incorporate electronic parts, it is thus impossible that the ice warrior, who has been entombed since the last ice age, is anything other than an alien being. Minutes later, an emergency meeting distracts the staff and no-one notices that the ice block has completely melted and the creature has begun to show signs of life. The creature becomes mobile, knocking Jamie unconscious and taking Victoria as a hostage.
The ioniser planning meeting is interrupted by the Doctor’s news about the electronic connections on the warrior’s helmet. He concludes that if the creature is indeed alien then there could be a spaceship under the glacier which could be powered by atomic systems, and using the ioniser in that area could cause a massive explosion that would destroy the Base. The crew are just evaluating the data when a bemused Jamie bursts in and reports the creature has come to life and taken Victoria. An alert is issued throughout the base, but no extensive manpower can be spared for a search. Arden and Jamie are identified for a search party and prepare to go out and face the moving glacier.
The creature has now identified itself to Victoria as a Martian or Ice Warrior from the planet Mars, who has indeed been frozen for millennia. He insists Victoria help him find his ship and the other members of his crew. The aggressive and conquering being announces that with the extra troops he can decide whether to return home or stay and conquer the Earth. Varga steals some extra power packs from the medi-centre to help revive more of his warriors.
Penley has meanwhile helped Storr back to their hideout, where Storr’s injuries are determined so bad that Penley needs to go back to the Base to steal some medical supplies. He sees the Ice Warrior and Victoria in the medi-centre and follows them at a distance as they prepare leave the Base. They encounter Clent before leaving, and Varga leaves him unconscious and badly wounded. Penley tries to revive Clent and is found doing so by the Doctor, who has worked out he is the errant scientist. Penley has now got the medicine he needs and, once sure the Doctor is dealing with Clent, leaves the Base, despite’s the Doctor’s protests that he is needed to help with the Ioniser. When Jamie and Arden return from their preliminary search, the Doctor persuades them that with the prospect of more than one creature buried in the glacier they should now play for time. They postpone the next phase of the search until morning.
In the glacier Varga finds four of his frozen comrades and begins the process of reviving them, which takes all night. His lieutenant, Zondal, is given the task of creating defensive structures in their ice cave while the other Ice Warriors set about finding their missing craft and digging it out of the snow and ice. For some of the time Varga is observed by Penley, who is back tracking in the snow having used the medicine on his friend Storr. When Penley returns to Storr he is surprised to find a visitor, Miss Garrett, who implores him to rejoin the crew of the Base at this critical time. When other approaches fail, she tries to take him at gunpoint but Storr intervenes. Miss Garrett is sent away with Penley’s advice to “check the Omega Factor”. This advice is of assistance to the Doctor when Miss Garrett returns to Base. They test the new data in simulation mode. It proves that the Ioniser can be used more effectively, much to the joy of the newly invigorated Clent, who now hopes he can save civilization after all.
Back at the Base, the period of waiting is over. Jamie and Arden are again sent into the glacier, ostensibly to find the alien spacecraft rather than Victoria. They discover the Ice Warriors’ cave excavation and report this back to Base. Minutes later, they are ambushed and gunned down by the Ice Warriors, who leave them for dead. A vigilant Penley investigates and finds Arden has indeed perished, but Jamie is still alive. Penley decides to take him back to his home. Storr resents this intrusion and is not moved when Jamie awakes and believes his legs to be paralysed. Storr now decides to try and speak to the Ice Warriors himself, foolishly convinced they might be potential allies. Penley heads after him, warning of their ruthlessness.
Having failed to contact Arden, the Base personnel assume something bad has happened. Moments later, the video link comes alive, operated by Victoria, who has been allowed to escape and now tells them of the danger of the Ice Warriors and is probed by Clent about the propulsion system of their ship. Varga has cleverly now manipulated the Base into revealing their primary concern, and intends to use it as a weapon against them. This done, an Ice Warrior is sent to capture Victoria again and use her as bait to draw others to the ice cave near the grounded spaceship. They are right: the Doctor himself determines to go to the spaceship and rescue Victoria. Before leaving, he takes with him a phial of ammonium sulfate, which he estimates will be very noxious to aliens from a nitrogen based atmosphere such as Mars. However, Victoria flees ever deeper into the icy caves as shown in the picture and when the Warrior, Turoc, finds her he is caught in an avalanche and crushed – with Victoria alive but trapped in his hefty dead claw.
An examination of the engines of the Martian craft reveal them to be functional but yet low on fuel and therefore immobile. When the Ice Warriors encounter Storr they are, as predicted, deaf to his offers of help, especially so when he denounces scientists at the very time they want technical aid. Storr is killed but the gift of friendship he brought from the ice caves, Victoria, is permitted to live.
Meanwhile Penley has found the Doctor and taken him to his friend Jamie. He is able to determine Jamie’s paralysis is just temporary and then heads off to the Martian craft to find Victoria. Once there he offers himself as an envoy, leaving his communicator active so Clent can hear, and is allowed to enter the airlock and discuss the situation. With the glacier still moving and threatening to crush the spacecraft, the Doctor succeeds in getting Victoria released to him. He is less successful in persuading the warlike Varga that the Ioniser is anything less than a weapon that could be used against the Martians. The last thing the Doctor is able to establish using the communicator before Varga takes it is that Clent needs to use the Ioniser at some point, regardless of consequences. The Doctor is now marched to the core of the spacecraft, where he spots an ion reactor as a propulsion system. Varga now determines to attack the Base first before the Ioniser can be used, and orders his Warriors to prepare a sonic cannon.
Penley has meanwhile made a difficult journey (both emotionally and physically) back to Base, with Jamie on a motorised sled. There Leader Clent gives Penley a frosty reception, and they end up bickering about the way forward. Clent says he has decided to adopt the Doctor’s advice and use the Ioniser, even if the computer seems unconvinced of the merits of such a proposal. The Doctor and Penley’s new formula for ionisation has been tested in other Bases with great success and is scheduled for use in Britannicus in a few hours time.
Zondal has been given the task of arming the sonic cannon. With Varga and his other Warriors, Isbur and Rintan, focussed on this attack, the Doctor and Victoria use the opportunity to release the chemical solution in Zondal’s face. The Warrior collapses, his hand activating the sonic cannon as he falls, but fortunately the blast glances the Base. Varga now uses the communicator unit to appear directly to Clent, threatening to fire again unless the humans surrender. Clent knows the Base dome cannot survive another sonic blast and so suggests a peace meeting between the two sides. Varga, Isbur, and Rintan march into the Base and confront the humans in the Ioniser room but any progress when a demented technician, Walters, tries to shoot the Martians. Walters is terminated but his actions are seen in bad faith. Varga now begins to dismantle the Ioniser reactor to get the mercury isotopes he needs for his ship, not caring how this will affect the humans and the Dome. Without the ioniser, the glaciers begin to move forward.
The Doctor and Victoria have used a breathing space back at the Martian craft to readjust the sonic cannon so it will harm the Ice Warriors and not the humans. Similarly, Penley, who was not in the Ioniser Room, uses his freedom to alter the temperature and atmosphere controls in the Base so that they become very unpleasant for the Martians. Moments later the Doctor fires the sonic cannon, forcing Varga and his men to retreat from the Base on pain of death. He then fuses the sonic cannon before he and Victoria break away and head back to the Base, not passing the Martians heading the other way. The Doctor now revives the Base staff, who were rendered unconscious by the sonic blast, and works with Penley to recalibrate the Ioniser for use. There is a fifty-fifty chance it will explode when trained on a spacecraft with an ion engine, but this is deemed a chance worth taking, however cautious the advice of the computer is. Penley tells Clent to work without the advice of the computer, which has marshalled and dominated his thinking throughout. It is removed from the equation when the device overloads and Penley himself takes charge and starts the Ioniser.
The Martian craft is now loaded with mercury and begins to power up, but does not get far before it is destroyed in the blinding heat of the Ioniser trained on the glacier. It explodes without a chain reaction and thus, at a stroke, the Ioniser has solved both the problem of the Ice Warriors and the glacier. Their work done, the TARDIS crew slip away as some green shoots outside emerge through the melting snow.
Deborah Watling was unable to attend the complete recording of the final episode. Consequently, Victoria is asked (off-screen) to return to the TARDIS halfway through the episode.
All save episodes two and three of The Ice Warriors still exist in the BBC Archives. The four existing episodes were all found in BBC Enterprises' Villiers House property, in August 1988. They were only discovered as the corporation was in the process of moving out of the building.