It's the dawn of a new era for Martin Moone as he begins secondary school, and while he isn't impressed with his friends' talk about girls, he soon changes his tune when he meets art teacher Miss Tivnan, a vision at the blackboard who sparks a romantic rivalry between him and Sean. Meanwhile, Dessie has an important question for Fidelma.
Sean is the imaginary friend of 12-year-old Martin Moone, the youngest child of a family living in a small town in Ireland. Martin has a unique perspective on life aided by an imaginary friend.
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 judgement. With Sean's help, Martin negotiates life as the youngest in a chaotic, scatter-brained family.