After he witnesses a dazzling object in the sky, research student Richard Jarman learns that part of the greenbelt around London has been destroyed, ostensibly by fire. Convinced that the UFO was responsible, he tells the police who react with scepticism. When he discovers that similar incidents have been occurring across the globe, Richard becomes obsessed with discovering the cause.
He neglects his thesis and in his search for answers, becomes involved with an elderly vagrant and a nun who both witnessed the same phenomenon. Richard and the nun fall into the clutches of a shadowy religious cult whose terrifying leader believes that the destruction of the world's vegetation heralds the end of the world and the Second Coming of the Messiah. The planet is plunged into even greater crisis when spaceships descend and a host of seemingly indestructible beings invade some of the world's most populous cities.
When Richard and his girlfriend Fiona consult a famous medium, they believe they have discovered the motive for the invasion, but is it the right one?