After serving gallantly at Pearl Harbor, Navy Commander Don Winslow, and his pal and junior officer, Lieutenant "Red" Pennington, are assigned to the Coast Guard. There they are ordered to devote their activities to anti-fifth column work on the mainland. Don learns that The Scorpion, notorious alien enemy, is in the pay of the Japanese and is expected to lay the ground work for a Japanese attack on the Pacific coast. Constantly in peril and aided by Mercedes Colby, the daughter of a Navy Admiral, they investigate secret island-bases and battles with submarines and enemy airplanes.