It's the semi-final of Gareth Malone's contest to find the nation's most entertaining amateur a cappella group. To earn their place in the grand final, the four groups left standing must rise to a unique challenge - combine vocals, choreography and a killer concept to create a video with the potential to become an internet hit. Before the cameras even roll, one group has to contend with losing their leader, while a bizarre take on rapping threatens to derail another. Once in the can, the videos will be screened to a live audience and a jury of a cappella experts. Their votes will decide who goes through to the final and which group will leave the contest for good.