With the Doctor behaving rather oddly after his regeneration, the Brigadier finds a scientific research station under attack after a robot steals the plans for a new disintegrator gun. Could the members of the mysterious Think Tank be responsible for the giant invader? How sinister are the intentions of the shadowy Scientific Reform Society? More importantly, can the Doctor recover in time to save the Earth from Atomic War?
Originally transmitted 28th December 1974 – 18th January 1975, this four part adventure heralds the beginning of the Tom Baker era for Doctor Who.