Sean (Chris O'Dowd) is the imaginary friend of 12-year-old Martin Moone, the youngest child of a family living in a small town in the rural Ireland. Martin, aided by his imaginary friend, has a unique perspective on life. His imagination comes into play both in his childish drawings, which come alive through animation, and in the ridiculous schemes he comes up with, against Sean's better judgment. With Sean's help, Martin negotiates life as the youngest in a chaotic, scatter-brained family.