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A demonstration by unilateral nuclear disarmament protesters (Campaign for Nuclear Disarmament) in London is interrupted when one of the protesters is violently killed. British security forces learn that a militant group attached to CND has been planning a significant act of terrorism for the near future. The person killed during the protest demonstration was an undercover intelligence officer who had infiltrated the terrorists. The Commanding Officer of the Special Air Service, Colonel Hadley, suggests a new line of inquiry for the investigation.
At the SAS's headquarters in Stirling Lines, two foreign officers arrive to train with the SAS. Captain Hagen is a member of the U.S. Army Rangers and Captain Freund is a member of GSG9. They are taken to the Close Quarter Battle house and witness an SAS room entry assault first hand. Colonel Hadley introduces them to the SAS man playing the "hostage", Captain Peter Skellen, and informs them they'll be with Skellen's troop, consisting of Baker, Dennis and Williamson. During an exercise in Brecon Beacons, Hadley and Major Steele discover Skellen's troop torturing Hagen and Freund. Skellen is dismissed from the Regiment. The torture along with Skellen's dismissal are a ruse to repaint Skellen as a disgraced former SAS operative, and Hagen and Freund are innocent victims in the scheme.
Skellen meets with his intelligence contact, Ryan, who informs Skellen that he should meet Frankie Leith and Rod Walker. Leith and Walker run The People's Lobby, the militant group believed to be planning the act of terrorism. Skellen tells his wife Jenny that he'll be going away for a while on a mission. Skellen then visits a bar frequented by members of The People's Lobby and begins a conversation with Leith. Skellen initiates a romantic relationship with her, to the annoyance of Walker and his cohorts Helga and Mac. Leith takes Skellen to the organization's offices and introduces him to the group. Leith appreciates Skellen's SAS background, and offers him a job as security consultant to the PL; she also allows him to move in with her. To strengthen his cover story, Hadley informs Hagen and Freund of Skellen's location. The wronged men attack Skellen at Leith's home and inflict a severe beating; as a result Leith's few remaining doubts about Skellen vapourise, but Walker and his cronies still are not fully convinced. Helga observes Skellen meeting the same unknown individual (Ryan) in various locations. Their scrutiny intensifies even more when Walker and his associates witness Skellen meeting with his wife and daughter. They use photos from their surveillance to convince Leith that Skellen is not all he seems. Walker orders Helga to kill Ryan, cutting Skellen's link to Hadley. Hadley has no choice but to trust Skellen's abilities to uncover the group's plans and escape alive. He orders police protection for Skellen's family.
Despite his official advisory capacity, Skellen is denied details about the upcoming operation. On the day of the operation, Leith and Walker instruct Helga and Mac to take Skellen's family hostage. Leith uses this to blackmail Skellen into unconditional co-operation. The terrorists and Skellen arrive at the US Embassy in a hijacked coach. Wearing stolen US Air Force uniforms, they gain entry to the secure compound and take the US Ambassador, US Secretary of State, US Commander in Chief of Strategic Air Command, British Foreign Secretary, along with their wives and the embassy staff, all hostage. Hadley and his police counterpart, Commander Powell, arrive at the Embassy to receive the demands of the terrorists: Unless a US Nuclear Missile is launched at Holy Loch Naval Base, all the hostages will be killed.
Meanwhile, Dennis and Neil arrive at Skellen's home with two fellow SAS troopers. They set up in the attached house next door, using sensing devices to covertly observe Helga, Mac and their captives through the wall.
Currie questions Leith's motivations, and Leith responds that her ultimate goal is the disarmament of the whole world. This opens a debate about method and political philosophy that only antagonizes the terrorists. Skellen manages to separate himself from the group by feigning a need for the toilet. He uses a torch to signal Hadley via Morse code, telling him to attack at 10 a.m. while Skellen creates a diversion. Hadley can't get permission for an SAS attack because the British Home Secretary insists that Powell resolve the situation through negotiation. As the tension mounts inside the embassy, a mistake by one of the terrorists causes the death of the General Potter, C-in-C of SAC. This allows Powell and Hadley to get the permission for their assault.
Frayed nerves cause Helga and Mac to squabble with their charges, allowing the SAS operatives next door the opportunity to burst through the wall and kill both Helga and Mac.
As the SAS mount an assault on the embassy the terrorists panic. Skellen overpowers and kills two terrorists. The SAS, deploying from helicopters, force open doors and enter through windows. As they methodically clear the embassy, Skellen kills more terrorists including Walker. Skellen joins with Baker and his troop to search for Leith, as the other troopers bundle the hostages to safety. When Skellen hesitates to kill Leith on sight, Major Steele kills her before she can kill Skellen. The Ambassador thanks the troopers as they leave the embassy. Skellen and his troop apologise to Hagen and Freund and explain the reason for their actions, and make peace. Skellen departs on one of the helicopters with his colleagues.
An on-screen list of notable terrorist incidents appears over the closing credits, accompanied by a rendition of The Red Flag.