Delivered in soothing ‘50s retro animation along the lines of The Iron Giant, Monsters vs. Aliens is an effective animated film that, while not offering up anything particularly new or revolutionary, delivers an engaging story that should keep kids entertained while providing moms and dads the occasional knowing chuckle. Bride-to-be Susan (Reese Witherspoon) awakens to a dappled Modesto sunscape, ready to take the leap of faith that will make her the wife of TV anchorman Derek (Paul Rudd). Her faith in his ability to break out of their small media market and become a major player in television news is, of course, misplaced; even younger viewers immediately sense that when Derek effuses how there’s no “I” in Team Derek, he’s full of beans.

The natty bridegroom shows his true colors during the wedding, when the hapless Susan turns into a giantess, courtesy of a prenuptial kick to the head from a falling meteor. Within seconds, the military is on the scene, subduing sobbing Sue and transporting her to a top-security facility in which the government houses monsters like Missing Link (Will Arnett), blobbish B.O.B. (Seth Rogen) and a former scientist whose work on making humans as resistant to destruction as roaches went too far, Dr. Cockroach (Hugh Laurie).

Poor Susan. It is a credit not just to Witherspoon’s perpetually sunny personality but to her very real acting chops that we feel the enormity of the character’s sadness and devastation at losing the world as she knew it and becoming Ginormica. Initially appalled by her prison mates, she soon discovers their appeal and appreciates their friendship. As it turns out, the monsters have a job, which is save the world from alien invasion, and here is where the script takes mildly humorous potshots at 1950s science-fiction movies. The chief villain, voiced with vivid aplomb by Rainn Wilson, is Gallaxhar, whose intelligence just might not be all that he thinks it is. Between scenes of the destruction of the Golden Gate Bridge and the monsters’ first crack at showing what they’ve got, Ginormica begins to understand, then utilize, the immense power she now has. One of the best things about Monsters vs. Aliens is how Susan is transformed from brown-haired doormat to platinum-blonde wonder woman, and her final confrontation with Derek is particularly empowering for all the little girls out there who might have otherwise looked to the Pussycat Dolls for inspiration.

The film takes it easy with contrived backstories—Gallaxhar wants Susan because the alien material that transformed her is exactly what he needs to gain ultimate power—but the filmmakers throw in some funny and unexpected bits. Stephen Colbert voices a suave bolla president who uses an electronic keyboard to appease the invading aliens, and Kiefer Sutherland’s tribute to Dr. Strangelove’s Buck Turgidson includes the delicious observation that the monsters’ prison is “an x-file wrapped in a cover-up and deep-fried in a paranoid conspiracy.” While Monsters doesn’t go too heavy-handed in the message department, it does invoke subtle ethical questions about our use of science and biomedicine, not to mention warfare. Clearly designed with an eye to sequel potential, Monsters vs. Aliens will no doubt spawn a next installment, which one can only hope is no worse than this harmless entertainment.