In London, a kidnapping is attempted on the Mall. Jack Ryan manages to disrupt the attempt by force, saving the Prince and Princess of Wales, along with their infant firstborn son, from the attackers. These attackers are members of an ultra-radical Irish terrorist group splintered from the Provisional Irish Republican Army.
Ideologically, their group subscribes to Maoism, and they receive support from Libya. They are known as the "Ulster Liberation Army," or ULA. Ryan incapacitates one of the ULA members, Sean Patrick Miller, whose father was killed in an incident with the Royal Ulster Constabulary in 1979, and whose girlfriend had been killed by a stray bullet from British Army forces. Miller is captured and sentenced to life imprisonment for murdering the royal driver, who was exposed to gunfire due to a fault in the vehicle's bulletproof glass. However, he is freed by his ULA compatriots while being transported to prison, and allies in the Mukhabarat el-Jamahiriya arrange for the party to be smuggled by sea to their training camp in the Libyan Desert.
The ULA, assisted by an African-American terrorist group, later goes after Ryan and his family. This is in part an act of revenge, but primarily it is done because the ULA seeks to reduce American support for the rival Provisional Irish Republican Army. The assassin sent to kill Ryan is intercepted before he manages to complete his task, but Ryan's pregnant wife, Cathy, and daughter, Sally, are injured when Miller causes their car to crash on a freeway. They are flown by helicopter to the University of Maryland Medical Center.
After the attack on his family, Jack accepts an offer from the Central Intelligence Agency (CIA) to start working as an analyst at the agency's headquarters. Later, the Prince and Princess of Wales come to visit Ryan at his house in Maryland, but this gives the ULA another opportunity to strike. In a sneak attack posing as telephone repairmen from Bell Atlantic, they try to kill Ryan and his family, and once again target the Royal Family for kidnapping. Although several guards, including Secret Service personnel, are killed, this second attack also ultimately fails.
After a firefight and chase, Ryan, his friend Robby Jackson, and the Prince manage to kill or capture the terrorists. They receive assistance from local police, US Marines, and sailors from the U.S. Naval Academy, managing to apprehend or kill all the terrorists. The FBI Hostage Rescue Team is sent by helicopter, but by time they arrive, the emergency is nearly resolved. Although the ultimate fate of the terrorists is not stated in Patriot Games, the next novel reveals that all who survived the attack were condemned to death and executed by gas chamber.
Ryan arrives at Bethesda after the final arrest to be with Cathy for the birth of their son, who will be god-parented by Robby Jackson and his wife, as well as the Prince and Princess of Wales.