Dramatic World War II action aboard a U.S. submarine is the setting for the only film in which Ronald Reagan and Nancy Davis star together. Based on a true incident, submarine Commander Casey Abbott (Reagan) leads a daring mission to enter the heavily mined Tsushima Straits to sever a vital link between the Asiatic mainland and the Japanese home islands. Abbott charts the dangerous waters, destroys an enemy island installation and sinks thousands of tons of enemy shipping. In the course of the operation, Abbott teaches his second in command, Lt. Commander Landon (Arthur Franz), the important difference between command decision and emotional reaction. Landon is put to the test when he must orde ia crash dive which leaves Abbott swimming alone in the trackless sea. When rescued, Abbott is cared for by nurse Blair (Davis), and now he too must test his own emotions.