In the Kentucky backwoods, hillbillies Jepson and Tully see a UFO crashland and an injured Alien stagger out. Foster is in America already and teams up with operative John Gillman to search in a Mobile. But when they find Jepson and Tully's shack, Gillman is shot. The Alien has told them he is a top secret test pilot, and the SHADO men are foreign spies. Having successfully captured Foster, the Alien kills Tully but is attacked by a deranged Jepson. The hillbilly locks Foster in and sets fire to the shack. Foster smashes through a window and follows Jepson, who has returned to the UFO. But Foster's approach triggers the self-destruct and Jepson is killed in the explosion.
Straker and Foster watch from a helicopter as the base nears completion. The Aliens have learned of its existence, and the construction is on full alert until operational. In the Mexican foothills near Veracruz, revolutionaries plan the overthrow of their dictator government. They have learned of a shipment of arms which they plan to steal. Meanwhile, a UFO has evaded the Interceptors and lands near the revolutionaries camp. The Alien is taken to the leader but the UFO self-destructs. The leader realises the Alien may be able to help steal the entire ship, rather than just the arms, and soon a convoy heads for the quay. Straker has been alerted, and Foster despatched to investigate. But the guerrillas have already found their target, and the Alien pilots the ship away with leader Guevanda. When Paul investigates the stolen ship in a helicopter, he is shot down and left adrift in a dinghy. By now Guevanda has realised the Alien has its own agenda, heading for the new SHADO base. In a fight, Guevanda is killed and the Alien mortally wounded. But it sets the ship on an flaming and explosive collision course before dying. With seconds to spare, Sky One flies in and destroys the ship, saving the base.