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. Jack gives a lecture at the Royal Naval Academy and then walks out to meet up with his wife Cathy and daughter Sally.
Meanwhile, Cathy and Sally are sightseeing near Buckingham Palace. The two come upon a British Royal Guard, and Sally tries to get the guard to react by doing an improvised tap dance in front of him. She's impressed when the guard, trained to ignore distraction, has no reaction to her antics.
As Sally and Cathy walk away from the guard, en route to rendezvous with Jack, they walk by a stolen cab, in which sit three Ulster Liberation Army (ULA) terrorists: Kevin O'Donnell, the driver, as well as Sean Miller (Sean Bean) and his younger brother Patrick. The three are loading their guns as they prepare to carry out a scheduled ambush on Lord William Holmes, British Secretary of State for Northern Ireland and a member of the British Royal Family (the cousin of the Queen Mother). As they finish loading their weapons, another terrorist, Annette, radios to them that Lord Holmes is leaving the palace with his wife and son.
As Lord Holmes's car leaves the palace, the terrorists' car falls in line behind him. They follow the car, headed for the ambush point. As they drive, O'Donnell and the Miller brothers don ski masks.
Meanwhile, Ryan is about to cross the street to meet with his wife and daughter. He arrives at the same time that Lord Holmes's car is passing through the area.
Suddenly, another cab, parked on the side of the road, driven by Annette, pulls out and swerves in front of Lord Holmes's vehicle. Simultaneously, O'Donnell's vehicle swerves to a stop behind the car. The Millers and another accomplice jump out and quickly plant a bomb underneath the chauffeured car's engine. Ryan sees what's going on and hastily gets his family to cover just as the bomb is detonated. Moments later, Sean and another accomplice begin shooting into the car, riddling Lord Holmes's driver and bodyguard with bullets. The onslaught does not kill Lord Holmes, who is currently using his body to shield his family. Sean marches over to the rear door and orders Lord Holmes out of the car at gunpoint. Ryan, unable to stand being just a bystander, immediately breaks cover, runs up behind Miller, and disarms him. As Ryan grabs Sean's pistol, he is shot in the left shoulder by another terrorist. Ryan returns fire, hitting one of the other terrorists in the shoulder. Patrick attempts to flee, but Ryan fatally shoots him in the chest. Seeing two of the royal guards racing towards the scene, the remaining terrorists quickly get back in their cars and drive off, leaving Sean to be captured when the authorities arrive. They retreat back to a warehouse, where they change out license plates.
Ryan wakes up in a hospital, recovering from the wound to his shoulder. He's visited by a representative from the US consulate and officers of the London police and questioned about his heroics. He's told about both Kevin O'Donnell, the leader of a violent ULA splinter group and Sean and Patrick Ryan, who were both orphaned at a young age and came under O'Donnell's care and tutelage. Sean is an especially dangerous and violent sociopath who shares O'Donnell's radical views about British rule of Northern Ireland.
Ryan is called to testify against Miller as a witness. Though the defense attorney questions Ryan's involvement in the incident, citing that he's not a field agent with the CIA or any other organization that would operate in his manner, Ryan holds his own. Miller has an outburst as Ryan is led out of the court chamber and vows revenge. Subsequently, Miller is convicted on all charges and sentenced to life in prison. Ryan is awarded the order of Knight Commander of the Victorian Order, and eventually returns to the United States. When they arrive, Cathy tells him she's been having morning sickness and that's she's pregnant.
On the possibility that Miller's ULA comrades might try to liberate him, the authorities take no chances. To that extent, they set up decoy convoys on the day that Miller is to be transferred to Albany Prison on the Isle of Wight. However, O'Donnell's associate, Annette, is able to get information about the convoy that carries Miller from a man named Dennis Cooley, a rare books dealer and mole. Miller's motorcade is in transit with the officer in charge of Miller's arrest and incarceration, Robert Highland, when it comes to a stop at a drawbridge. The raised bridge, however, is a trap meant to block them in. Moments after the convoy comes to a stop, Miller's comrades attack, using rocket launchers to blow up the cars at the front and rear of the convoy. They then march up to the prison van and order the guards out at gunpoint by taking the bridge operator hostage and threatening to shoot him. One of the guards hesitates and is shot fatally. The other officer and Highland are pulled out by O'Donnell, thrown on the ground and forced to lie on their stomachs. O'Donnell accuses Highland, who is Irish, of turning against his own country and people. O'Donnell then hands Miller a pistol, with which he coldly executes both officers and the bridge operator by shooting them in the backs of their heads. They then depart the scene.
Miller and his companions flee on a tramp steamer from Britain to Libya, to prepare for 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 on a short mission targeting Ryan and his family.
Miller travels to the United States, accompanied by Annette, comrade Ned Clark, and a fourth henchman. Annette and Clark travel to the United States Naval Academy to ambush Ryan as he's leaving work. Ryan notices Clark idling nearby. Clark casually walks away, but Ryan clearly unnerved, then gets even more nervous as he hears a car engine starting. His suspicions are well warranted-as he continues walking down the street, he looks in the reflection of a parked van's rearview door and sees that Clark, and the stolen car driven by Annette, are following him. Ryan pretends not to see them until he's walking past a parked panel truck, at which point he ducks behind the truck. Clark draws a silenced pistol and prepares to corner Ryan, but Ryan attacks him first. The two fight hand-to-hand and Clark quickly gets the upper hand, throwing Ryan to the curb. He grabs his pistol and is about to shoot Ryan when he's shot dead himself by a US Marine sentry. Annette quickly drives away in her Jeep. Ryan, realizing that Miller is going after Cathy and Sally, jumps into his car and drives off.
Simultaneously, Miller and the other henchman follow Cathy as she picks up Sally from school, with Miller riding shotgun. They prepare to follow the two, but are temporarily held up by a crossing guard. Once the crossing guard clears, they pursue her to US Highway 50. Ryan tries to frantically call Cathy on his car phone, but she is busy answering phone calls from the hospital. As he's trying to connect with her, Miller and his henchman recklessly weave in and out of traffic to catch up with Cathy. Ryan eventually gets through to Cathy and tells her to get off the highway and find the nearest police station. Unfortunately, just as the call ends, Miller's van comes charging up on her using the left shoulder. Miller moves to the backseat of the van, grabs a submachine gun, and slides open the door. Another car momentarily delays him, but once he has a clear aim, Miller opens fire, spraying Cathy's car with bullets. A multicar pileup ensues as Cathy loses control of her car and crashes head-on into a concrete barrier. Ryan, who has found himself stuck in traffic headed in the other direction, sees the smoke from the crash and realizes the worst.
Cathy and Sally are airlifted to a nearby hospital for treatment. Cathy will recover quickly and her pregnancy is unaffected. Sally, however, is in very bad shape and her chances are 50/50. Ryan eventually finds out a few hours later that Sally will recover but it will take time. Late one night Ryan receives a phone call from Miller himself. Miller taunts Ryan, suggesting that Jack can't protect his family and he'll never find Miller. Enraged, 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). His old contact, Marty Cantor, teams him up with a small group that specializes in the ULA and other Irish terrorist groups.
Ryan's tireless work leads him to conclude that Miller has taken refuge in a training camp, one of many located in Libya. He also determines that a recent new arrival at the camp may Dennis Cooley, who'd escaped the London police when he discovered they were using surveillance devices in his book shop. Ryan, through an IRA spokesman, Paddy O'Neil (Richard Harris), also determines that the woman with long red hair that he saw in London and in near the Annapolis Naval Academy, is the same woman who works for O'Donnell and is in one of the Libyan camps.
Ryan makes his recommendations to his superiors at the CIA. Greer invites Jack to a live feed of an SAS strike team attack on the camp. Everyone in the camp is eliminated but unbeknownst to the CIA and Ryan, Miller and his companions have already fled the camp and are 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 by stealing one of the boats they'd arrived in at the cliffs near the Ryans home. A furious boat chase in a heavy storm on Chesapeake Bay ensues, with Miller, Annette and O'Donnell in pursuit.
The FBI Hostage Rescue Teams are scrambled to pick up Holmes. Upon realising that Ryan is trying to lead 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 when it hits a rocky reef.
Credits roll just after Caroline Ryan learns the gender of the child she is going to have, and before she tells Jack and Sally.