This send-up of The Pirates of Penzance was clearly inspired by the success of Papp's Pirates on Broadway. The adaptation is extremely loose; we see far more of Trevor Farrant and Peter Sullivan than of Gilbert and Sullivan. The protagonists — Kristy MicNichol and Christopher Atkin — were teen idols at the time this was made, and the film was clearly intended to appeal to a juvenile audience. If you can ignore what you know about the original opera, the film is a clever romp in its own right. It has far less to do with G&S than the Papp production (which, despite all the criticism it has received, is comparatively faithful to the original). Ironically, this version beat Papp to the big screen and was considerably more successful. 

Do yourself a favor and try to find a copy of The Pirate Movie, no matter how bad you've heard it is. If you know the real Pirates backwards and forwards and inside out, especially if you've performed in it, the movie is good for laughs (and groans, much the way you'd react to an atrocious pun). It's so bad it's good, much as Plan Nine From Outer Space is considered. It would be a complete mystery and have little, if any, entertainment value for someone with no knowledge of the real Pirates, though. For example (spoken, not sung as in the real Pirates).

A Gilbert & Sullivan Discography