An elite Special Air Service (SAS) officer, Major Peter Skellen, infiltrates a radical anti-nuclear group known as the People's Lobby. Posing as a disgraced soldier, he earns their trust and gains access to their inner circle, discovering their plans for a major act of terrorism. As he gathers intelligence, he forms an uneasy but necessary bond with Frankie Leith, the group's passionate leader, while maintaining contact with his SAS handlers. Tensions rise as Skellen works to balance his undercover persona with his true mission, navigating the deadly stakes and ideological fervor of the activists.

The situation becomes increasingly volatile when the People's Lobby takes a prominent U.S. diplomat and his family hostage at a high-profile diplomatic function. Skellen must relay critical information to his SAS team while keeping his cover intact. The group's radical actions draw international attention, escalating the urgency of the SAS mission to prevent widespread bloodshed. As pressures mount, Skellen faces deep moral and ethical dilemmas, challenging his resolve and dedication to duty while attempting to avert a catastrophic crisis.