“Monsters Inc.” is one of my favorite Pixar movies, filled with wit, imagination, and heart. This prequel is a lot of fun, still very funny and wildly imaginative, but a little hollow where the heart should be.

One problem Pixar just can’t solve is that a prequel has to end before the original begins. “Monsters Inc.” has a brilliant premise: there’s a monster world fueled by the screams of frightened children. The monsters themselves are terrified of humans, even a toddler named Boo.

There is a power factory that sends them each night into children’s bedrooms. The monsters have to scare the kids without being seen by grown-ups and get home without being “contaminated” by contact.  By the end (SPOILER ALERT) the monsters have discovered that children’s laughter is an even better energy source, and the audience goes home feeling happy and reassured.  But a prequel has to stick with the idea that scaring children is a worthwhile goal, indeed it needs us to get on board with the idea that we should root for the characters to be really good at it.  We know Mike and Sully will end up as friends. So the sweetness and the dramatic tension are dialed down.

Once again, our heroes are Mike (Billy Crystal), the anxious one who looks like a green beach ball with arms and legs and one great big eye, and Sully (John Goodman), the giant polka-dotted furry guy who thinks it all comes naturally and he does not need to work.  They both pick the prestigious “scaring” major, under the stern eyes of Dean Hardscrabble (Helen Mirren, impeccable as always) and Professor Knight (Alfred Molina).

Fans of the original will be intrigued to find that in the beginning, Mike and the chameleon-like Randy (Steve Buscemi) were roommates and friends.  How that turns to rivalry while the initial enmity between Mike and Sully turned into professional partnership and personal BFF-dom is the story of the film, with some overtones of “Animal House,” “Harry Potter,” and every ragtag group of underdogs movie you’ve ever seen.

Mike is the one who studies all the time.  Sully is the party animal who thinks that he can get by on charm and talent.  Both find themselves kicked out of the program, with just one chance to get back in.  If they can be a part of the team that wins the intramural games, they can get back in the scaring program and become professional human child scarers.  They will have to work together — and bunk together — with the oddballs and rejects at the bottom of the school’s social hierarchy, the members of a fraternity known as Oozma Kappa (OK).  Their fraternity house is the home of one of the members, with his mom as their RA and chauffeur.

The frat brothers are adorable, especially the two-headed Tracy/Traci (voices of Sean Hayes and Dave Foley), and a fuzzy purple log-shaped guy named Art who looks like a Muppet reject (Charlie Day).  Art is a new age philosophy major who eagerly presses his fellow OK-ites to try dream journals.  Don (Joel Murray) is a middle-aged guy trying for a new career (apparently there’s a recession in Monster-world, too).  None of these monsters is especially smart or strong or fast or scary.  They have to compete against the fearsome athletes of Roar Omega Roar (ROR), let by the arrogant Johnny (Nathan Fillion).

There are some exciting and funny moments in the competition, especially a too-knowing obstacle course where the teams have to avoid a truly terrifying foe: human teenagers.  The monster-ification of the classic college movie developments is a lot of fun.  In making sure each team has a quorum, Johnny sneers, “We count bodies, not heads.”  Tracy/Traci only counts as one.  Of course, the struggle to be liked by the cool kids is the same whether you’re a person or not.

They did not want to go for the usual ending here, which is admirable, but the result is surprisingly downbeat and disquietingly know-nothing.  If is not the loud, over-done “Cars 2,” it is also not the expansive, transcendent “Toy Story” sequels.  Second-rate Pixar is still better than most of what is out there, but we expect more.

P.S. As always, the movie is preceded by a marvelous animated short from up-and-coming Pixar-ians.  This one echoes last year’s “Paperman” romantic (and meteorological)  theme, with blue and red umbrellas finding each other in a rainy city.

Parents should know that this film has some mild peril, bullying, insults, and hurt feelings.  Characters cheat and have to pay a penalty.

Family discussion: Why didn’t Mike and Sully get along at first? How were they different? What was good and bad about the fraternities in the movie and how are they like groups you know?  How do they make a deficiency into an advantage?  How can you?

If you like this, try: “Monsters, Inc.,” “Sydney White,” and “The Lawrenceville Stories”