This documentary looks back to last September, when David Walliams swam the length of the River Thames. Having previously completed swims across the English Channel and the Strait of Gibraltar, the comedian's latest feat proved to be his greatest test of endurance yet, during which he battled hypothermia, caught a bacterial infection and even saved a dog from drowning. The film follows his challenge, from entering the water at Teddington Lock to reaching the steps of London's South Bank, eight days and 111,352 strokes later, for an emotional reunion with wife Lara Stone.He combats sickness, bone-crunching cold, pain and tiredness to raise more than ?1 million.