From the beginning of this third series, the character of Joannie Cunningham, very skillfully played by Erin Moran, is noticeably turning from a child into a young woman and the writers acknowledge that fact in this episode when Joannie develops a major crush on a clueless Potsie.
The one-sided romance begins when Potsie sings a song to her at Arnolds and she is convinced that he shares her feelings which means that she starts sending anonymous love notes. For his part, Potsie is intrigued by the unknown identity of his admirer and keeps wondering aloud who she might be? Finally, when the two arrange to meet and Potsie realises that his admirer is his best friend's little sister, he has to let her down very gently.
A really good episode that makes us all remember what it's like to have a crush on someone who doesn't know we exist. Great acting by Anson Williams and Erin Moran too, they both show a lot of skill in this episode.