WHO needs a movie about a tyrannical infant - or an infantile tyrant - anyway? You might be surprised to learn that you do.

Although its advertising campaign seems to promise little more than an animated comedy about a bratty baby in a business suit, "The Boss Baby" (adapted from the 2010 book by author and illustrator Marla Frazee) is a sweet adventure tale about sibling rivalry that ultimately becomes a moving tribute to family and brotherhood.

Seven-year-old Tim (voice of Miles Christopher Bakshi) is an only child, basking in the undivided attention of his parents. He has a wild imagination, dreaming up elaborate imaginary rescue scenarios involving pirates and rocket ships. But this perfect life is upset by the arrival of a new baby brother (Alec Baldwin), who appears not in the usual fashion, but has been sent to Earth via a heavenly sorting procedure that divides newborns into loving family types and "management" babies, raised in cubicle farms and emerging into the world wearing three-piece suits and carrying briefcases.

From Tim's (admittedly unreliable) perspective, his unnamed middle-manager sibling uses play dates to conduct meetings, fielding business calls on a Fisher Price toy telephone. (The film takes place in an indeterminate time period - perhaps the early 1990s - in which there are computers, but no cellphones.)

Boss Baby demands complete attention from his parents, leaving Tim feeling neglected. This transforms "The Boss Baby" from a single-joke movie to a story with a deeper, more universal resonance. After all, who among us - even an only child - hasn't felt the pang of abandonment, if not sibling rivalry, at some point?

Tim and his little brother are bitter rivals until Boss Baby reveals an important secret mission to stop what poses the direst threat to what the film posits is babies' already tenuous hold on parental love: puppies.

Although the film's character design is, for the most part, undistinguished, its vivid backgrounds are informed by both pop-up books and quirky midcentury design, and the script (by Michael McCullers of both "Austin Powers" sequels) drops pop-culture references that range from "Teletubbies" to "Apocalypse Now." The animation concept for the film's rival companies - Baby Corp. and Puppy Co. - is impressively futuristic.

Yet what really drives the film is the central relationship, a fraternal dynamic that, despite being based on a flight of fancy, is more convincing than many live-action family comedies manage to be. While "The Boss Baby's" corporate adventures are clearly the product of a child's overactive imagination, the film's lessons - about how both Tim and Boss Baby must learn to come to terms with each other - are very valid.

Director Tom McGrath ("Madagascar") strikes a fine balance between humor and sentiment, never losing sight of the tender reality that fuels childhood fantasy. Inventive and heartwarming, "The Boss Baby" is a lot more grown up than it looks.