One September morning, the Soviet Navy submarine Sprut finds itself too close to the New England coast. Its curious captain, eager to see North America, inadvertently runs aground on a sandbar near Gloucester Island, home to a mere 200 inhabitants. Rather than risking an embarrassing international incident by calling for help, the captain dispatches a nine-man landing party led by his zampolit, Lieutenant Yuri Rozanov, to search for a motor launch to assist in freeing the submarine.
Upon arriving at the residence of Walt Whittaker, a vacationing playwright from New York, the situation escalates. Walt is eager to whisk away his wife Elspeth and children, Pete and Annie, now that the summer season is drawing to a close. Young Pete informs his father about the imposing presence of armed Russians nearby, but Walt is initially skeptical and questions their intentions. When confrontations arise, Rozanov reveals their true identity and brandishes a weapon, although he assures Walt that no harm will come to his family as long as they cooperate and offer their station wagon.
As the plot thickens, Elspeth hands over the car keys, but the predicament worsens when the station wagon runs out of gas, compelling the Russians to abandon it and resort to walking. In a twist of fate, they commandeer an old sedan owned by Muriel Everett, the local postmistress. This action sets off a wave of panic as Alice Foss, the talkative telephone switchboard operator, starts spreading rumors of an air attack, leading to chaos on the island.
Police Chief Link Mattocks and his deputy Norman Jonas struggle to maintain order as a citizen militia led by Fendall Hawkins threatens to erupt. Meanwhile, Walt, Elspeth, and Pete manage to overpower Alexei, another Soviet officer, when their neighbor Alison Palmer, a babysitter for Annie, unexpectedly shows up. This event provides them an opportunity to escape, but the Russians are close on their heels, leading to more misunderstandings and confrontations.
In a dramatic turn, Walt finds himself captured again, this time at the telephone exchange. After subduing Mrs. Foss, he becomes tied up alongside her, while the Russian crew, now dressed in civilian clothes, makes their way toward the submarine. In a poignant subplot, Alexei and Alison develop a bond amidst the chaos, sharing a kiss that hints at the possibility of love in desperate times.
Once freed from their bindings, Walt and Mrs. Foss team up with Elspeth and Pete, intent on resolving the misunderstanding. In a tense encounter, Walt narrowly avoids harming Rozanov, further complicating their precarious situation. As tensions rise, the submarine floats free from its sandy trap, heading toward the island's main harbor, where a standoff with the townspeople ensues.
With Rozanov acting as a translator, the Russian captain threatens to attack unless his missing sailors are returned. In a heart-stopping moment, two boys clamber up the church steeple, and one accidentally falls, only to be saved by the combined efforts of the American islanders and Russian submariners forming a human pyramid.
In the end, peace is brokered, but trouble looms as Hawkins contacts the Air Force. Nevertheless, the submarine receives an escort of villagers in boats as it departs. In a touching farewell, Alexei bids goodbye to Alison, while the Sprut escapes the harbor just as two Air Force jets approach, ultimately free to venture into deeper waters and safety.
