According to the long tradition of breakout projects, “Son of Rambow,” the second feature film by the British writer and director Garth Jennings, should either show its creator “cutting loose” or, alternatively, “finding his footing.” Such would be the natural follow-up to Mr. Jennings’s first effort, the unwieldy studio adaptation of the Douglas Adams cult classic “The Hitchhiker’s Guide to the Galaxy.” The 2005 film was greeted with predictable bemusement despite (or because of) its fidelity to the book’s dry absurdity.

The conventional wisdom is half right: Mr. Jennings starts out gangbusters, only to turn disappointingly unimaginative. The second half of “Son of Rambow” has the feel of a hack sequel to the first half, losing a loony-but-credible touch for childhood and friendship in order to go through the motions (quirky though they may be) and wrap things up. Still, on the whole it’s better than most studio comedies.

Set in England in the 1980s, “Son of Rambow” concerns the liberation of two young lads by means of a camcorder. Lee (Will Poulter) is a belligerent outcast, repeatedly kicked out of class, who wants to enter a short-film competition. Will (Bill Milner) is a meek, slight, perpetually jacketed oddity belonging to a family of Plymouth Brethren that has never watched television. The pair meet in a school hallway (Will has been excused from a screening, Lee booted for the usual). Lee, a winning little grifter, recruits his credulous schoolmate to be the heedless stuntman in his self-made action movie, filming him hurtling down hills or being tossed into a lake. Will supercharges the project with a “Rambo” fixation that he acquired after glimpsing the movie, and that is partly based on the Henry Darger-esque fantasy-world he keeps in a thick notebook.

Will’s father is out of the picture, as you might have guessed, while for Lee, the problem is a single mother gallivanting about Europe and the nasty older brother who rules the roost in the meantime. What’s appealing (at this point) about “Son of Rambow,” besides seeing waifish Will being catapulted through the air, is that the two boys are, in truth, a bit damaged, so the pair’s growing bond and mutual respect carry real warmth. When they first meet, Lee is pathologically quick in cheating Will out of his watch, while Lee’s fantasies about saving his father (who was killed in action) are nightmarishly delirious and abrupt, and delivered by Mr. Jennings with imagination-appropriate animations and special effects.

Where Mr. Jennings falters is in the rest of the boys’ world, where the jokes and hugs are forced. The student body is in thrall to a French exchange student, Didier (Jules Sitruk), who’s dressed to the nines in period pop androgyny. He becomes the headliner for the Rambo film, with Will’s blessing, and the production amasses a growing crew. The ’80s references pile up, culminating in a prolonged party that spotlights a joke about the candy Pop Rocks.

A little of all this goes a long way, and it’s easy to feel as betrayed as Lee, who is gradually elbowed out of the production. After all, the comedy of their amateur movie doesn’t grow from references to the original “Rambo” or insufficiency before it (à la Michel Gondry’s “Be Kind Rewind”): For the young creators, it’s just right. Maybe it’s no surprise that by the time the film winds down and everyone reconciles, both “Son of Rambow” and the movie within the movie are shells of themselves. In the excerpts of the finished product, Will and Lee speak in an exaggeratedly wooden, ill-paced manner, going for an easy joke that doesn’t match their earlier performances.

Will’s problems with his mother (Jessica Stevenson) are also resolved tidily, as they sweep aside the entreaties of their overly attentive pastor, and Lee’s mean but adored brother finally shows a glimmer of affection that’s kept from corniness with a self-consciously goofy twist.

Some of “Son of Rambow” was inspired by Mr. Jennings’s own childhood experiences shooting miniature epics in Essex, England, around the same time that the real-life ragtag band behind “Raiders of the Lost Ark: The Adaptation” were making Mississippi their set, and countless other junior Spielbergs were no doubt doing the same. Though “Son of Rambow” is smarter and more fun than the typical fare, one wishes Mr. Jennings and company could have preserved the spirit of those earliest adventures from beginning to end.