"Fun Size" is a peculiar hybrid, a crude, frequently subversive PG-13 comedy that, as a Paramount/Nickelodeon co-production set on Halloween, has been marketed to little kids of an appropriate trick-or-treating age. Parents of pre-adolescents will be in for some unpleasant surprises if they aren't aware walking in that the material targets teenagers more than 8-year-olds. Adding to the confusion is an adorable 8-year-old's misadventures happening in tandem with the central 17-year-old protagonist's equally hijinks-heavy experiences, and an overriding tone that uneasily wavers between juvenile slapstick, highly questionable wayward elements treated as punchlines, and an unexpectedly moving drama about a family struggling to move on after the death of a loved one. Only the latter portion works, and it is handled with such a gentle yet forthright maturity it's a shame the rest of the movie gets in the way.

The October 31 holiday is treated as a pretty big deal in high school senior Wren's (Victoria Justice) Midwest hometown of Cleveland, and she's hoping to take advantage of it. Indeed, things start to look up when the hottest guy in school, Aaron Riley (Thomas McDonnell), invites her to his Halloween party. Unfortunately, mother Joy (Chelsea Handler) has other ideas. Since she's going to a party of her own with new, younger boyfriend Keevin (Josh Pence), she wants Wren to watch little brother Albert (Jackson Nicoll) and take him trick-or-treating. Albert hasn't spoken since their dad's death the year before and the responsible Wren, who dreams of going to NYU after graduation, isn't about to let him down. With socially conscious best friend April (Jane Levy) tagging along for moral support while trying to come up with any way possible that they can still make it to Aaron's house before the night's out, Wren suddenly finds herself in potentially a whole heap of trouble when she loses Albert. Desperate to find him, she convinces neighborhood friend Roosevelt (Thomas Mann) to borrow his moms' car to help look for him. Meanwhile, Albert meets a lonely convenience store worker named Fuzzy (Thomas Middleditch) and has soon set out on a mission with him to win back ex-girlfriend Lara (Abby Elliott). Yes, that would be 8-year-old Albert. Helping out an adult stranger. And getting into his car. At least in this regard, the film acknowledges how uncomfortable the situation is.

"Fun Size" is the directorial debut of Josh Schwartz, one of the writers and creators of the long-running CW series "Gossip Girl," and was written by Max Werner (Comedy Central's "The Colbert Report"). It's wildly uneven to the point of feeling like two or three different movies cobbled together. Wren is a nice and likable heroine with an uninhibited goofball side, and she's played with charisma by Victoria Justice (TV's "Victorious") in her first major film role, but never, in even the slightest way, does she truly act like a teenage girl whose little brother is missing. For all she knows, he could be dead in a ditch somewhere, but the script continuously turns Albert into a storytelling gimmick and afterthought (save, that is, for when the narrative follows him). Just because the viewer is aware of where he's gone and that he's kind of safe—first to a convenience store, then into the car of a grown-up man who takes him under his wing while trying to work out relationship issues—doesn't mean Wren does, and by the time she's foregoing her search in order to get serenaded by Aaron Riley at his party, it's more than clear that "Fun Size" is not existing in the real world so much as in a teen girl's fantasy.

From destruction at a fast-food drive-thru that leads to a giant plastic chicken humping a car, to a run-in with a middle-aged man who creepily sits on the curb and watches the little children go from house to house around him, to the appearance of a possibly psychotic character named Jurgen (Johnny Knoxville)—Fuzzy's ex-girlfriend's highly unlikely new boyfriend—who abducts Albert, locks him in a room in his house, and refuses to release him to Wren, the film is all over the map, journeying to very dark places while playing most of them for laughs that never come. After a while, the film starts to irritate and rub wrong, despite an admittedly capable Halloween atmosphere worked up by the production design, art direction, and soundtrack cues (many of them related in some way to autumn and/or the holiday).

Just as hope appears to be lost, "Fun Size" does a 180 turn in the last twenty minutes, so drastically different it's as if a far better writer and director have taken the reigns from imbeciles. There is a wonderful monologue delivered by Joy to the party host's parents (Holmes Osborn and Annie Fitzpatrick) when the raucous twenty-something shindig in the basement gets to be too much for her. Expressing frustration and grief over suddenly losing her husband, the stress of raising two children on her own, the trepidations she has letting her daughter move across the country to college, and the ridiculousness of a woman in her mid-thirties hanging out with kids barely out of college, Joy finishes up her unloading of emotions with the perfect last remark—honest and funny and authentically human just as Chelsea Handler (2012's "This Means War"), in a surprisingly moving and nuanced performance, has brought the audience on the edge of tears. A few scenes that follow are equally compelling and dramatically sound—Wren's and Albert's visit to their father's grave comes to mind—as they're not too pushy and, temporarily, put the hokey comedy on the backburner. It helps to have an appropriate musical accompaniment, and the use of Passion Pit's "It's Not My Fault, I'm Happy" and Milo Greene's "Autumn Tree," among others, are effective, going a long way in righting a few of the film's wrongs. Unfortunately, just not enough of them.

"Fun Size" will likely perplex most viewers as much as it did this one. The film is too adult for tweens and younger, and too juvenile for older crowds. People of Wren's and April's age may be best drawn to it, but the advertising campaign has done a poor job of catering to such a crowd. It is destined to find its most receptive audience when it's out of theaters and available on video, where its coarser material and perturbing character actions will be more palatable. Indeed, it's a movie that's all over the place, bad one minute and refreshingly non-stereotypical the next. BMOC Aaron Riley, for example, isn't vain or stuck up, but a nice guy—even if he's not quite right for Wren—while the picture more often than not listens and cares about its characters (characters, that is, not named Jurgen). "Fun Size" has a stronger finish than start—and really, since when did Chelsea Handler become such a fine actor, in addition to being an entertaining comedienne and talk show host?—but it's not quite enough to soften the irresponsible behavior on display from a protagonist who really should know better. Thank goodness this is a comedy first and foremost, because otherwise poor Albert would have wound up in a garbage bag by the end of the night.