While on a camping trip with his son and his co-worker, George is nervous because the co-worker keeps on playing pranks and later mistakes Harry for a monster.
It's Halloween and Harry finally has a chance to get out. However, the Hendersons' neighbour, who is allergic to animal hair, becomes suspicious of Harry's "costume."
Brett thinks a primitive mask can cure his ailing love life by casting a spell on Darcy's Swedish au pair Uma, and it seems to work; but his date with her is spoiled by Harry, who is strangely obsessed with the mask as well.
Harry scores high on Ernie's IQ test, which changes the way that everyone relates to Ernie, especially when his teacher Ms. Hall comes to visit and encounters Harry.