Gareth Malone kicks off a singing contest with a difference, as he begins his search for the nation's most entertaining amateur group who can sing a cappella - without instruments or backing tracks, they must rely solely on their voices. In this episode, Gareth mentors the first four groups singing to stay in his contest. He'll strive to bring out the best performances from a London community choir, an Essex gospel group, six Southampton students and a group of female barbershoppers from Portsmouth, before they face a live audience and a jury of world-class a cappella experts. The jury's votes will decide which groups go through to the next stage and who will leave the contest for good.