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.

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.

What really drives the film is the central relationship, a fraternal dynamic that is more convincing than many live-action family comedies manage to be.

Director Tom McGrath (“Madagascar”) strikes a fine balance between humor and sentiment, never losing sight of the tender reality that fuels childhood fantasy.