Tearjerker doesn?t really think outside the box

"Martian Child" is one soggy slab of sentimental uplift, but it doesn't pretend to be anything else, and there's some honor in that. Or maybe I've just been cutting John Cusack slack ever since he hoisted that boom box over his head in "Say Anything."

He plays David Gordon, a successful science-fiction author, a widower, and - if we're to believe the script adapted by Seth E. Bass and Jonathan Tolins from a novella by David Gerrold (himself a successful sci-fi writer who penned the classic "Star Trek" episode "The Trouble With Tribbles") - a lifelong misfit. Since Cusack remains the most congenially sane of working movie stars, that's a stretch, but no matter.

Mired in grief but looking to move on, David decides against the better judgment of his agent (Oliver Platt) and sister (real-life sibling Joan Cusack) to adopt a child. Not just any child, mind you, but a little boy so socially maladjusted he lives in a cardboard box. Dennis (Bobby Coleman) believes he's from Mars and that the sun's rays will fry him to a crisp. In the real world, he'd probably be diagnosed a schizophrenic and dosed with heavy meds, but the world according to Hollywood sends him home with David for 100 minutes of bonding and life lessons.

Coleman is adorable without being obnoxious about it, and he plays Dennis as a whispery wounded bird; you want to reach in that box and take him home yourself. The best scenes in "Martian Child" let David and Dennis knock around the former's well-appointed house, slowly opening up to each other. Cusack's earnest befuddlement works well in such a setting, and he makes each line of dialogue a little aria of seriocomic distraction. Faced with this boy who fell to earth, David creates an in-home alternate environment. "Think of it as a bigger box," he says.

The film's most radical notion is that it takes an eccentric to raise an eccentric, and that the rest of the village would do well to leave them alone. I'm not sure whether this is true, but it feels good, and that's what "Martian Child" is selling. Snarking away on the outsides of the house are a meanie social worker (Richard Schiff), the harried and very funny Joan Cusack, Platt's spineless agent, and a dragon-lady publisher played by Anjelica Huston. "I want Harry-bloody-Potter in space," she rages at David, who's way over deadline. "Why can't you just be what we want you to be?"

Any frazzled parent recognizes that sentiment, whether it ever slips out or not, and in its moist and obvious way "Martian Child" recognizes the difficulty of remaining unique in a lockstep world. A tougher movie wouldn't downplay Dennis's real emotional damage, but a tougher movie probably wouldn't bring on Amanda Peet as David's late wife's free-spirited best friend whose protective presence gives the boys some needed space.

Because we have places to get to, "Martian Child" eventually cooks up a few late-inning plot crises and a teary, hug-filled climax. The strain shows. The heart of this mawkish yet touching drama is far simpler: A good man and a troubled boy working hard to connect. Only in the movies does the work ever stop.

Ty Burr can be reached at tburr@globe .com. For more on movies, go to boston.com/ae/movies/blog.