In the vibrant spring of 1943, Lt. (j.g.) John F. Kennedy, portrayed by Cliff Robertson, finds himself requesting an assignment to the PT boat base located at Tulagi in the scenic Solomon Islands. Amidst a backdrop of scheming yeomen and the constant threat of Japanese air raids, he soon encounters Commander C.R. Ritchie, played by James Gregory, a crusty maintenance officer who indicates the high standards expected of him. Shortly after, he meets his new commanding officer, Lt. Alvin Cluster, brought to life by Grant Williams. Determined to take command of a PT boat, Kennedy accepts responsibility for the battered and crewless PT 109, which is moored under a net of camouflage. Ritchie gives him just one week to transform this wreck into a formidable fighting machine, and thus begins Kennedy's intense mission.
With his new executive officer, Ensign Leonard Thom, brilliantly portrayed by Ty Hardin, and a crew of nine enlisted men, Kennedy embarks on a daunting journey to whip PT 109 into shape. Despite the clock ticking down, the efforts bear fruit, and PT 109 soon meets Ritchie's rigorous standards. Sent out on trial maneuvers, Kennedy and his men quickly discover that returning last to the base means enduring long hours at the refueling pier. Meanwhile, they face warnings from their engineers regarding the dangers of reversing engines at high speeds. As luck would have it, the men learn of an imminent military operation: the invasion of the nearby island of Rendova, designated to serve as a new base.
Just as refueling is about to conclude, Cluster orders PT 109 on an emergency mission to evacuate Marines from Choiseul. Engaging in a fierce battle, Kennedy and his crew provide cover fire, heroically coming to the aid of a stricken platoon when their evacuation vessel is struck by a grenade. However, during their retreat from enemy fire, PT 109 finds itself running out of fuel, and the incoming tide threatens to drag it back within range of Japanese artillery, only to be rescued by another PT boat bearing a tow line. Unfortunately, three of Kennedy's crew sustain injuries and are replaced just before the squadron is reassigned to the safeguarded base at Rendova.
Despite Ritchie's initial orders to remain behind at Tulagi, he asserts his influence over the flotilla commander and successfully arranges to join Kennedy and the crew of PT 109. The perilous journey to Rendova is not without its dangers; a sudden attack by a Japanese fighter plane catches them off guard. Ritchie passionately urges the gunner to allow him a shot, ultimately managing to down the enemy aircraft with remarkable accuracy.
Upon reaching Rendova, Kennedy's eagerness leads him to overestimate his resources. After executing one high-speed reversal too many, PT 109's engines falter, crashing into a maintenance shed at the pier where Ritchie sternly reprimands him. However, the situation escalates as they find themselves amid a protracted air raid by Japanese bombers. During the chaos, Kennedy reunites with an old acquaintance, George "Barney" Ross, played by Robert Culp, who joins PT 109 as they man a 37-mm anti-tank gun secured to the foredeck.
As PT 109 sets out for a late-night patrol to intercept the dreaded “Tokyo Express” before it can reach Guadalcanal, Kennedy engages in a conversation with his torpedoman, Andrew Kirksey, who harbors an ominous feeling of impending doom. The darkness enveloping Blackett Strait conceals the dreadful approach of a Japanese destroyer, which suddenly rams PT 109 amidships, devastatingly splitting it in two. Surviving crew members are thrown overboard, many severely burned and injured amidst the chaos. With bravery radiating from every pore, Kennedy and the uninjured crew swim through the flaming wreckage, desperately rescuing those in peril while their boat barely stays afloat.
The following morning, the survivors cling to the remnants of PT 109. In a bold move, Kennedy resolves to swim to the nearest uninhabited island, leading his men on a treacherous three and a half-mile journey to safety. Upon reaching land, they narrowly escape detection by a nearby Japanese patrol barge, yet fail to attract the attention of a passing American reconnaissance plane. Though many crew members succumb to despair, Kennedy remains resolute. Over the next two nights, he and Ross venture further into the open sea in search of friendly patrols.
Back at the command staff in Rendova, hope dwindles for the lost crew. Undeterred, Kennedy steers his men toward the island of Olasana. It is here that two native Solomon Islanders discover the stranded Americans. Communicating without words, Kennedy skillfully carves a message on a green coconut, enlisting their help to deliver it to Rendova. The next day, another native canoe arrives, this time with a message from Reginald Evans, a nearby Australian coastwatcher, who transports Kennedy to meet him. Despite the dangers, Kennedy's persistence pays off; Evans radios to Rendova with news of the survivors.
The culmination of their harrowing ordeal arrives when Kennedy coordinates a rendezvous with PT 157 at Paporan Island. His crew, still hidden in the underbrush, emerges as the specter of salvation approaches. Even with each crew member entitled to a month of survivor leave, Kennedy stays grounded in his mission, deciding against a break and promptly taking command of PT 59, a converted gunboat. With five former PT 109 crew members by his side, his journey continues as they gear up to face the challenges that still lie ahead.
