You know a comedy franchise has hit rock bottom when, within the first 15 minutes of a movie, you’ve been forced to watch an obese man receiving an enema and a 2-year-old boy projectile vomiting into the face of his dad. So it goes with “Little Fockers,” the third and (let’s all cross our fingers here) presumably last installment in the series about the put-upon nurse Greg Focker (Ben Stiller) and his ex-CIA agent father-in-law Jack Byrnes (Robert De Niro).

Greg and his wife Pam (Teri Polo) are now happily, if busily, married in Chicago, with their twins’ third birthday fast approaching. But our hero faces temptation in the form of Andi Garcia (Jessica Alba), a knockout pharmaceutical sales rep who wants Greg to become a spokesperson for a new male sexual enhancement drug called Sustengo. (And yes, they beat that joke about how she shares a name with the actor Andy Garcia far into the ground.)

Jack, meanwhile, is facing mortality: He’s recently suffered a heart attack, which — for reasons that make about as much sense as everything else in this slapdash effort — only Greg knows. The assorted plot points converge in one of those “I-can’t-believe-I’m-actually-watching-this” moments, when Jack has a negative reaction to Sustengo, requiring Greg to inject his aroused private parts with a shot of adrenaline.

As much as De Niro probably earned to endure “Little Fockers,” he should have asked for another $5 million for this scene alone.

The plot of “Little Fockers,” of course, is really just an excuse to assemble a bunch of familiar faces and allow them to schtick it up. At least on that score, “Little Fockers” obliges. Barbra Streisand, as Greg’s sex therapist mother, and Dustin Hoffman, as his “menopausal” father, endearingly hover around the edges, and Owen Wilson — who’s long been the best part of this series — once again returns as Pam’s ex-boyfriend Kevin, a Zen-preaching investment banker who’s forever in the throes of a spiritual crisis.

New to the mix — though large chunks of their performances seem to have been left on the cutting room floor — are Laura Dern, playing the headmistress at a tony preschool where Greg and Pam want to enroll the twins, and Harvey Keitel, as Greg’s do-nothing contractor.

Yet there’s absolutely nothing at stake here: We know Greg would never cheat on Pam, and we know Jack’s suspicions will eventually be allayed and father-in-law and son-in-law will once again come to an uneasy truce. Clearly in a state of desperation, director Paul Weitz and screenwriters John Hamburg and Larry Stuckey pile on the puerile sight gags and struggle to come up with new ways to humiliate a bunch of performers who have been humiliated plenty in “Meet the Parents” and “Meet the Fockers.”

By the time Greg and Jack have a bloody fistfight in a children’s playground ball pit, “Little Fockers” is no longer running on fumes — it’s empty.