On the coast of north Devon, Ross is joined by Emily Turton, as he prepares to dive down on the wreck of sunken steam ship Iona II. The ship 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. Ross hopes his night dive might reveal what they wanted back so badly but the wreck is shrouded in dense low visibility which hampers their investigations.