Jack Ryan (Ford) is on a "working vacation" in London with his family. He has retired from the CIA and is a Professor at the US Naval Academy. They witness a terrorist attack on Lord William Holmes, British Secretary of State for Northern Ireland and a distant member of the British Royal Family (the cousin of the Queen Mother). Ryan intervenes and kills one of the attackers, Patrick Miller, while his older brother Sean (Sean Bean) looks on. Ryan is badly wounded.
The remaining terrorists flee and leave Miller to be apprehended by the police. Whilst recovering, Ryan is called to testify in court against Sean Miller, who is part of the Ulster Liberation Armya breakaway group of the Provisional IRA. Ryan is awarded a knighthooda KCVO: Knight Commander of the Victorian Orderand eventually returns to the United States.
While being transferred to Albany Prison on the Isle of Wight, Miller's escort convoy is ambushed by his ULA comrades who kill the police officers, and he escapes from custody. Miller and his companions flee Britain for North Africa to plan their next kidnapping attempt on Lord Holmes. Miller however, cannot shake his anger towards Ryan for killing his younger brother and persuades several members of his entourage to accompany him to the United States to murder Ryan and his family.
Ryan survives an attack outside the United States Naval Academy. Simultaneusly, Miller and a colleague attack Ryan's wife and daughter as they're driving on a busy highway. They crash; both Cathy and her daughter are severely injured.
Enraged over the near-loss of his family, Ryan decides to go back to work for the CIA, having earlier rejected the appeal of his former superior, Vice Admiral James Greer (James Earl Jones).
Ryan's tireless work leads him to conclude that Miller has taken refuge in a training camp, one of many located in Libya. A SAS strike team attacks and kills everyone in the camp while Ryan looks on through a live satellite feed. But unbeknownst to Ryan, Miller and his companions had already fled the camp and were on their way to the US to stage their next attack on Lord Holmes.
Lord Holmes decides to visit Ryan at his home to formally present his KCVO. With the aid of Lord Holmes' traitorous assistant, Miller's group tracks Holmes to this location, kills the Diplomatic Security Service agents and Maryland state troopers guarding the house, and attempts once more to kidnap Lord Holmes. Ryan leads Holmes and his family to safety while he attempts to lure Miller and his companions away from his home.
The FBI Hostage Rescue Teams are scrambled to pick up Holmes. Upon realising that Ryan is leading them away from Holmes, Miller's companions try to persuade Miller to turn around, but an enraged and deranged Miller kills his terrorist companions and continues his pursuit of Ryan. Ryan and Miller fight hand to hand; Miller is killed when Ryan impales him backward on a boat anchor, and his body is obliterated in the subsequent explosion of the craft.
Credits roll just after Caroline Ryan learns the gender of the child she is going to have, and before she tells Jack and Sally.