The year is 1944, Atze and Samuel occasionally operate on the black market, but mostly keep quiet and wait for the Americans to come. When the last German submarine, the U-900, is sent on a final mission, Atze foolishly messes with the general of this mission and has to go undercover. Together with his friend Samuel and the actress Maria, who disguises herself as a man for this purpose, he therefore smuggles himself onto the U-900 and pretends to be the captain. Since he has no idea about seafaring and gives increasingly strange orders, the crew soon becomes suspicious.