Dan Snow dives in search of underwater caves where people might have lived thousands of years ago, Steve Backshall scales the sea-cliffs to find out how the St Kildans got food when times were tough, and Kate Humble goes to find one of the wonders of the natural world. Together the team discovers why the last St Kildans left their island home less than 80 years ago.

Kate Humble goes beneath the waves to find out how St Kilda's wildlife is faring underwater. Dan Snow investigates a World War II plane crash and attempts to trace the story of human habitation on the islands back beyond the Vikings. Steve Backshall tries to find out who or what is harming the islands' most secretive inhabitant, the rare Leach's Storm-petrel.

Steve collects food the St Kildan way by abseiling down a 400 foot cliff, Kate finds out how this season's baby puffins are getting on, and Dan discovers the islands' Viking history. But when Steve and Dan set out to row to the island of Boreray, all does not to to plan.