Following his regeneration, the Fourth Doctor becomes delirious and falls unconscious in front of Sarah Jane Smith and Brigadier Lethbridge-Stewart of UNIT. The Brigadier has Lieutenant Harry Sullivan take the Doctor into his care. Soon, the Doctor wakes, eludes Harry, and tries to sneak off in his TARDIS, but the Brigadier and Sarah stop him, convincing him to help in finding the culprit in the theft of top secret plans for a disintegrator gun. The Brigadier takes the Doctor to the Ministry of Defence advanced research centre where the plans were stored, where the Doctor observes crushed flowers and a rectangular footprint. UNIT attempts to protect factories where critical parts for the gun are manufactured, but the culprit is able to outwit them, including burrowing up underground to collect the final component necessary for the weapon.