A new, more advanced generation of aliens known as the Morthren arrive on Earth. Dr Blackwood is saved from a kidnapping by a renegade soldier called John Kincaid. After executing the original alien invaders for failing to conquer humanity, the Morthren aim to destroy Blackwood's team with a clone of Colonel Ironhorse.

When a severe heat wave strikes, the aliens cut off the city's water supply and exploit a local reverend to drive humans into worshiping the Morthren Eternal.