Retired commander of the United Nations' Intelligence Taskforce, and long-time associate of the mysterious time traveler the Doctor, Brigadier Alistair Lethbridge-Stewart faces the toughest battle of his military career when he is embroiled in a plot unwittingly set in motion by university chancellor Victoria Waterfield, herself a former companion of the Doctor, to take over the Earth by an evil alien entity called the Great Intelligence, aided by its ferocious robot Yeti cohorts. Can the Brigadier defeat this menace to the Earth without the Doctor's help?