Set in the weeks after Resistance: Fall of Man, Resistance: Retribution follows the story of former British Marine James Grayson. After a personal tragedy - he is forced to kill his own brother inside a Chimeran conversion center - Grayson goes on a vendetta to destroy every conversion center he can find. Eventually Grayson learns that his efforts have been futile... in Western Europe the Chimera have evolved a new method of converting humans to aliens. The European resistance - the Maquis - enlist Grayson's help, and he joins Cartwright and Parker in Operation Overstrike. The war to retake the European continent has begun.

Set two months after the events of Resistance: Fall of Man, British Royal Marine Lieutenant James Grayson lead his team on a raid in a Chimera conversion center; Grayson stumbles upon a horrible sight: his long-missing brother; Johnny Grayson, who is already infected with the virus and half-converted into a Menial. Forced to follow the protocols regarding infected soldiers, Grayson makes the impossibly painful decision to shoot and kill his brother. As a result, Grayson suffers a nervous breakdown and deserts from his team, and launches his own personal vendetta against the Chimera. Grayson succeeds in destroying approximately 26 conversion centers, but is eventually captured and taken into custody for desertion. A capital offense; Grayson is put on trial to which Stephen Cartwright, now promoted to Major and Grayson's commanding officer, testifies against him. Grayson is found guilty and sentenced to death by firing squad.