In 2008 body-builder Mark Rix died of a heart attack, seemingly caused by a surfeit of steroids. Now a sword is found in his old lock-up, which decapitated an unknown man just before Mark died. Mark was a member of Imperium, a group re-enacting life in Ancient Rome and Steve and Sasha learn from Imperium members Julia Kane and Tim and Tessa Dugdale that he believed himself to be Marcus Rex, incarnation of a gladiator. Forensic scientist Fiona Kennedy identifies the murder victim as a Romanian gangster who sold illegal steroids and the team deduces that Rix used Imperium's foreign dates to import them. After confessing to Fiona that his estranged wife Sarah is in a secure unit, having tried to kill him and his daughter, Danny goes undercover at Imperium, where he recognizes Julia as another undercover cop. Eventually the team learns the truth about both men's deaths from Julia before Danny goes on a date with Fiona.