A puny guy has volunteered to train as a frogman (USN UDT, precursors to USN SEALs), but is immediately nicknamed "Sardine" by guys called "Shark" and "Whale". He recalls being bullied and coddled as a kid, but looks as if he'll wash out. Then, as the team trains at raft-righting, a shark comes gunning for his teammates, and "Sardine" dives in the way!
The team kills the shark, and Sardine and the guys graduate as frogmen. Flying to the Pacific, their DC-4 is shot down, then rammed by a Japanese cruiser. They're picked up by a PBY. At base, they get their first mission: to destroy an enemy ship menacing a beach landing. They fly in, but bailing out, become targets for Japanese night fighters!
The parachuting frogmen hurl TNT and blow up the night fighter. Swimming for the island, they dodge depth-charges. A sub net guards a sub berth. Shark sets a charge on a sub as Sardine and Whale continue with the mission. Then Whale is lost as he fights off Japanese frogmen. Sardine moves on and sinks the targeted cruiser with an explosive charge. And is thrilled to learn Shark and Whale made it. "We can't call that kid Sardine any more!" "Mission completed!"