Playing a mentally challenged person must be a seductive prospect for an actor who wants to show his range, but though it’s something that might earn plaudits (as “Rain Man” did Dustin Hoffman and “What’s Eating Gilbert Grape” Leonardo DiCaprio), it’s a risk that’s ordinarily best declined, because the projects associated with such roles are usually banal and mawkish. That’s certainly the case with “I Am Sam,” an almost brutally manipulative piece awash in schmaltz that writer-director Jessie Nelson (along with co-author Kristine Johnson) has fashioned for Sean Penn. Penn is a resourceful and fascinating performer, but as the title character here–a man with the mind of a seven-year old who goes to court to challenge the state’s decision to remove his young daughter Lucy (Dakota Fanning) from his care–he delivers a showy, flamboyant turn that reeks of technique put to poor use. Quite simply, it’s one of the worst performances he’s ever given.

Penn isn’t really responsible, however. The simplistic script and Nelson’s sledgehammer direction don’t offer him any opportunity for subtlety. We’re obviously intended to side unflinchingly with Sam in his battle against social worker Margaret Calgrove (Loretta Devine) and badgering lawyer Turner (Richard Schiff) to transfer guardianship over Lucy to suburban housewife Randy Carpenter (Laura Dern). But despite the fact that the narrative is heavily weighted in his favor, any reasonable viewer will seriously doubt whether a verdict in that direction would actually be in the child’s best interest. Sam and Lucy clearly dote on one another, but however charming he might be, one must wonder about the father’s ability to support the daughter financially or emotionally over the long haul. And despite the insistence of the script and the soundtrack that the Beatles provide the answer to everything (an especially irritating device), their message that “All You Need Is Love” seems distinctly insufficient in this context.

It doesn’t help that Sam’s supporters in his struggle are such a problematic lot. His closest friends are four other mentally-challenged guys, two of them (Doug Hutchison’s Ifty and Stanley DeSantis’ Robert) played by actors who, like Penn, are affecting their conditions and the other two (Brad Allan Silverman and Joseph Rosenberg) members of the L.A. Goal organization, which serves developmentally disabled adults. This quartet is used for alternately humorous and touching effect, very heavy-handedly. Sam’s other notable supporter is neighbor Annie, an agoraphobic musician who hasn’t left her apartment in many years. Dianne Wiest limns her with a mixture of maternal concern and neurotic trembling that doesn’t exactly make her an ideal witness when called to the stand.

These five, however, come across as positively normal when compared to Rita Harrison (Michelle Pfeiffer), the hard-driving attorney who reluctantly takes on Sam’s case pro bono to prove to her co-workers that she has a heart. Rita is excruciatingly badly written–naturally, she has a young son whom she neglects, and learns from her contact with Sam how to be a good parent–and she’s played even worse by Pfeiffer, who stutters, hems and haws her way through the part like a neurotic Diane Keaton on an exceptionally bad day. Hers isn’t so much a performance as it is a series of ticks, pouts and noble poses. But probably no actress could have saved the role when the courtroom sequences are so poorly constructed that they make Harrison appear to be an utterly incompetent attorney as well as a bad mother. Supporting players Devine, Schiff and Dern go through their paces professionally enough, but their efforts can’t stand up against the tidal wave of treacle that swells up and swallows everyone in the final reels. As Lucy (as from “in the sky with diamonds,” of course), Fanning is predictably cute as a button–one that’s pushed repeatedly to extract sniffles and tears from easily-manipulated viewers.

“I Am Sam” might have passed muster as one of those uplifting movies-of-the-week the networks churn out, especially for holiday showings (like, for example, “My Name Is Bill W.,” in which James Woods earned kudos as a mentally-challenged man in 1989). Its presence on the big screen, however, throws all its flaws into the sharpest possible focus. Unfortunately it seems a perfect distillation of the sort of stuff one might expect from the director of the relentlessly upbeat “Corina, Corinna” of 1994 and the co-writer of the sappy duo of “Stepmom” (1998) and “The Story of Us” (1999). Why Pfeiffer, who suffered through that latter disaster, would have allowed herself to be sucked into another of Nelson’s exercises in domestic bombast is a matter best left to a psychologist. This still-lovely actress really needs help in selecting roles more carefully; her recent choices have been poor indeed, and this one is surely the nadir. As for Penn, let’s hope his turn here is but an ill-advised aberration.