Greg James and his fellow celebrities battle the treacherous Grade 4-5 rapids of the Zambezi River during Comic Relief's Hell and High Water challenge. Soaked, bruised and paddling blind, Greg steers his raft through white water chaos while teammates Mel C and Jack Dee take an unexpected swim, Dara O Briain and Phillips Idowu go missing downstream, and the crew fights to stay afloat against gale-force currents.