After a tragic car accident in Lordsburg, New Mexico, investigators come across a strange machine part that piques their curiosity. Recognizing its potential danger, they send it to the Los Alamos National Laboratory, where scientists quickly realize it is a fragment of a highly advanced, portable nuclear weapon created by an unknown, possibly hostile, foreign power. This discovery raises serious alarms, prompting Paul Reagan, the chief of the Security Investigation Division (SID), to take swift action.
Agent John Ireland is assigned to uncover the source of these clandestine parts and to prevent them from being assembled into a deadly weapon. As the investigation deepens, more components find their way into the United States, and a pattern begins to take shape that points toward Marseilles, France. The plot thickens as authorities trace the smuggling operation from New Mexico to a U.S. Navy submarine in New London, Connecticut, where uranium is found welded to its hull, signaling the presence of a potentially nuclear threat.
Disguised as a naval officer, John Williams boards the submarine bound for France, undertaking undercover work to gather covert intelligence. In Marseilles, Williams uncovers a complex network involving civilians like Suzanne Dalbert as Margo Wayne and Peter Marshall as Leo Wayne, working alongside clarinet player Buzz Olin, played by Richard Avonde. They are connected to an elusive member of the submarine’s crew, all orchestrating the smuggling of weapon parts inside specially designed metal cases created by Pierre Neff.
A violent confrontation near the docks reveals suspicious activity involving Mike Connors as Lt. Magrew and Robert Foulk as Commander Jackson. Williams mistakenly believes these individuals are part of the illicit operation and orders their arrest. However, this leads to a betrayal by colleague Robert Hunter, who turns out to be an agent of the conspirators and captures Williams. The tension escalates as Williams is detained, prompting a series of tense moments in his effort to stop the impending disaster.
Back in Washington, D.C., Williams escapes captivity and directly contacts Reagan at the SID headquarters. It is then revealed that the entire operation was a simulated war game conducted secretly by the Department of Defense to evaluate national defense readiness against an enemy attack. Tragically, the suspicious figures—the Waynes and Buzz Olin—were not part of the simulation. They had actually succeeded in constructing four small cases, in addition to a large order of forty-eight, to smuggle a portable nuclear weapon into the country.
The countdown begins—less than 48 hours remain before the bomb is scheduled to detonate at 3 p.m. on Tuesday. Williams and his team race against time, tracking the conspirators to the San Francisco area. A desperate attempt ensues to hijack the weapon and bomb San Francisco from the air. During the chaos, Leo Wayne is killed, and William Bryant as Agent Jackson has only two hours to defuse the bomb. Meanwhile, Williams races towards Nevada, trying to disarm the device.
Despite his efforts, the bomb cannot be neutralized in time. With less than a minute remaining, the bomb is dropped from a plane and detonates over Frenchman Flat at the Nevada Test Site. The crisis is narrowly averted, and the film concludes with a voice-over stating, "…and three o'clock is just the middle of another afternoon in the life of a city.", highlighting the fragile balance of peace maintained by vigilant intelligence and timely action.
This story underscores the tense world of espionage and the relentless effort to prevent a global disaster, portraying both the high-stakes danger of nuclear proliferation and the heroism of those committed to defending their country from unseen threats.
