(It seems there is new attention for this movie, based on the filmmaker's lawsuit against J.K. Rowling. The main character is named Harry Potter Jr., there's a fantasy element to the film; but those are the only similarities.)
I hadn't seen this film since I was a kid, so long ago that it actually scared me. I didn't have a lot of hope for it to hold up, but I'm always willing to take a stroll down memory lane. Immediately I could understand why this scared me as a child. Within minutes of starting, the film has an evil ugly troll kidnap a little girl and take over her body, begin living with her family, including her brother, a little boy that I could easily identify with.
The Potter family has just moved to San Fransisco, into an eclectic old apartment building, where everyone knows everyone else, and everyone has interesting quirks. Harry Jr.; yes, that makes him Harry Potter, a good decade before the books, which is also highly distracting whenever they call him by his full name, which seems to be a lot; almost immediately notices that something is wrong with his sister. After watching a cheesy sci-fi film, he believes she's a pod-person, but soon discovers the truth, thanks to a friendly witch living upstairs from them.
Meanwhile, the Troll, hiding in plain site as a little girl, is quickly taking over the apartment building, thanks to his nasty ring that has poison needle on it. He's turning all the residents into various mythical creatures, because he's lonely. All their apartments start overgrowing with trees and plants, to better accommodate their transformed residents. Of course the Potter parents are completely oblivious to what's going on, and it's left up to Harry to save the day, with a little help from his witch friend and her weird mushroom boyfriend.
The film has some cool stop-motion effects, most notably when some giant vines start to take over the entire building, as a crowd of San Fransisco-ians watch. There's one creepy scene, at least from a child's perspective, towards the end of the film, where Harry is laying in bed, with his troll/sister lying in the bed next to him, and she keeps transforming back and forth between them. A pretty terrifying idea for any kid with a sibling sleeping in their room.
While I wouldn't call the film well made, it's too goofy for that; it is undeniably entertaining. In a so-bad-it's-good type of way. A kid can look past the bad acting and silly plot-line and enjoy it as a creepy fantasy story. Us grown-ups can enjoy it for the scene where all the puppet Trolls start singing in unison or a very young, half naked Julia Louis-Dreyfus starts frolicking around as some sort of fairy princess high on pixie dust. Troll is some good brainless fantasy/horror fan, if you enjoy it with a double order of cheese.
Don't confuse this with Troll 2, which has absolutely nothing to do with this film, beyond the name; and has gone on to gain some infamy as the "best worst movie". There's even documentary in the works about it all.