June 20, 1941. The U.S. submarine O-9 noses below the surface off the coast of New Hampshire, carrying a crew of 33, and is never seen again. Years of inquiries have failed to find answers to what caused this terrible tragedy - until now. In this exciting program, join Deep Sea Detectives Richie Kohler and John Chatterton, as they head out to sea with the National Undersea Research Center to take the first-ever video survey of the O-9 wreck in an attempt to solve this long-forgotten mystery. First, travel to the U.S. Naval Submarine School in Groton, Connecticut, where the team learns about some of the potential emergencies and worst-case scenarios submariners face while submerged. Next, investigate evidence of structural deficiencies on the O-9 before it was returned to service for the war, which could point to an explanation for the tragedy. Finally, experience the thrill of an up-close look at wreck, which provides crucial clues as to what might have gone wrong. Featuring cutting-edge technology, compelling archive material, and fascinating expert interviews, History proudly presents the ill-fated story of the Forgotten Sub of WWII.