The actor is joined by advanced technical diver Emily Turton in north Devon, as he prepares to dive down on the wreck of a sunken steam ship. Iona II was heading to America in the early 1860s, when it went down with a mystery cargo on board that the owners went to extensive efforts to recover in secret. The pair embark on a night dive with the hope it will reveal what the owners so desperately tried to covertly retrieve. Determined to get to the bottom of the mystery, Ross also meets with a maritime historian who has uncovered Iona II's shocking purpose. Ross and maritime archaeologist Mallory Haas head to Plymouth to dive on a newly discovered wreck, and to uncover a staggering artefact that links the site to one of the most controversial and censored periods in British history - the transatlantic slave trade.