Sing a Little Harmony is the story of Shion, a lovely and enigmatic girl who moves to Keibu High School and immediately becomes popular owing to her open-hearted attitude and remarkable athletic talent...until she is discovered to be an AI in the testing phase! Shion's objective is to make chronic loner Satomi "happy," but she goes about it in an unusual way: she serenades her in the midst of the classroom. Satomi and her childhood buddy, engineering nerd Touma, gradually warm up to Shion after learning that he is an AI. They, together with the popular and handsome Gocchan, the strong-willed Aya, and judo club member "Thunder," are increasingly affected by Shion's singing and seriousness, even as her pranks become more and more amusing.