In August 1917, Rosy Ryan, Sarah Miles, the daughter of pub owner Thomas Ryan, Leo McKern, grows restless in Kirrary, a quiet Irish village on the Dingle Peninsula in County Kerry. The place is deeply nationalist and openly hostile to the nearby British soldiers. Although Thomas outwardly supports Irish independence, he secretly acts as an informer for the British. Rosy yearns for excitement and begins a romance with the village schoolteacher, widower Charles Shaughnessy, Robert Mitchum. They marry and move into the schoolhouse, where his emotional reserve and lack of physical affection disappoint her.
Into this tension steps Major Randolph Doryan, Christopher Jones, a decorated British officer hampered by shell shock and a leg injury, who arrives to take command of the local army camp. He collapses in the pub during a flashback, and Rosy comforts him. The moment sparks a passionate affair that deepens during a secluded ride to the forest, while Charles grows suspicious but chooses silence.
While taking his students to the beach, Charles notices two sets of footprints—one belonging to Doryan and the other to a woman—leading to a nearby cave. He imagines Rosy and the Major together, confirming his suspicions. The footprints are also seen by Michael, a mentally disabled villager, who enters the cave and finds Doryan's Victoria Cross. Michael parades through the village with the medal, attracting ridicule. Rosy, disturbed by the scene, approaches him and unintentionally reveals her involvement. The villagers, piecing the events together, begin to suspect Rosy's infidelity.
One stormy January night in 1918, IRB member Tim O'Leary, Barry Foster, and his men arrive in the village seeking help in recovering a shipment of German arms. After assisting them, Thomas Ryan secretly alerts the British authorities. The villagers, including Ryan, join in the effort, unaware of his betrayal. When they succeed in freeing the truck carrying the weapons, British troops led by Doryan ambush the group. Doryan fatally shoots O'Leary before collapsing from another flashback. Rosy rushes to his side, further enraging the townspeople.
Later, Charles confronts Rosy, admitting that he had hoped her affair would pass but now wishes to separate. Rosy insists the affair is over, yet that night she visits Doryan again. Disheartened, Charles wanders to the beach and is found by the village priest, Father Collins, Trevor Howard. Meanwhile, a mob led by Mr. McCardle storms the schoolhouse, seizes Rosy, and publicly humiliates her, accusing her of betraying the arms operation. Thomas Ryan watches in silent remorse as they cut her hair, stopping only when Father Collins intervenes.
The next morning, Doryan encounters Michael on the beach and gifts him a cigarette case. In gratitude, Michael shows him a hidden cache of dynamite. Left alone, Doryan uses the explosives to commit suicide. Later, Charles and Rosy leave the village for Dublin. As Father Collins and Michael accompany them to the bus, the priest tells Charles that his remaining doubt mirrors his own: whether he and Rosy should remain together.
