The story of Frank Chalmers, an open water swimmer setting off on the challenge of his life. Having successfully swum the English Channel, Frank has been training for over a year in preparation for his toughest swim yet - to cross the Pentland Firth. This notoriously treacherous stretch of water separates the Orkney Islands from mainland Scotland and is home to some of the strongest tides in the world. The Firth has been named Hell's Mouth by seamen as its fearsome forces have earned it the reputation of being a graveyard for ships. Local seafarers, whose lifetimes have been dictated by these unpredictable conditions, team up with Frank and his wife Ros as they set out on this moving and unexpected journey.