Bert is given the role of playing pretend saxophone in a band with Ernie, Elmo and Abby Cadabby. But, Bert feels he needs a music expert to help him. Ernie mentions his music expert relative and brings Bert to Ernisimo's shop, where he gives Bert lessons on pretend saxophone.
Given Ernie's hasty mention of his uncle, the proximity of his shop to the street and his striking resemblance to Ernie, Bert assumes Ernisimo is merely Ernie in disguise. It is not until Ernisimo shows up at the pretend band's concert that Bert sees the two are actually separate entities.