After shore-leave in Lisbon for the crew of HMS Diana, Master-at-Arms 'Toby' Toberman makes an official complaint to the ship's Captain about the behaviour of an officer he saw in a casino. Following the ship's 'Sod's Opera' party, Toby is reported missing from the deck - presumed lost overboard. Was it an accident or suicide - or was he given the proverbial push?

With the ship now docked back at Portsmouth, Toby's friend 'Chalky' White is determined to find out the truth. He returns Toby's personal effects to his widow Jean, but the diary is still missing. They both attend the navy's inquest, but it leaves more questions than answers. Back at Jean's house, a letter from Toby arrives - posted when he was in Lisbon, and backing up his concerns about Kobahl and the casino. Chalky visits Kobahl's girlfriend - and also gets his Wren girlfriend Stephanie to do some digging.