Detective Inspector Andy Horton spots a police car outside Portsmouth Historic Dockyard. He's supposed to be on holiday, but his curiosity gets the better of him and he stops to investigate. Lying face down and sprawled out in Number One Dock is the lifeless body of naval historian Dr Douglas Spalding. Superintendent Uckfield is adamant that Spalding took his own life. But Horton's not so sure. Then another body is found in similar circumstances in the cockpit of a boat moored at Oyster Quays. Angry at his boss's inaction, Horton takes matters into his own hands.