The TARDIS materialises on Earth during the second ice age. At the Britannicus Base, like bases around the world, a computer and an ioniser device are used to keep the ice flows at bay.
A giant creature has been discovered buried in the ice, termed an Ice Warrior by one of the scientists, and it is brought into the base. It thaws, and the creature reveals itself to be Varga, commander of a Martian spacecraft that landed on Earth centuries ago and was entombed in the ice.
More Ice Warriors are revived. With Mars now dead they plan to take over Earth, and with Earth in the grip of a second ice age the conditions are perfect for colonisation.
The ioniser cannot be used for fear of exploding the Ice Warrior's ships engines. The computer, to which the base are dependent, can offer no advice in the situation. One of the scientists who supports the Doctor decides to risk using the ioniser, and the resulting explosion is only minor, only destroying the Ice Warriors and their ship, and keeping the ice at bay.
