An expert blend of silly fun, action that is mostly more exciting than scary, a few clever barbs, and some wow-worthy visuals make “Monsters vs. Aliens” the best family film in months.

“You’re glowing,” says the groom to his bride as they are about to be married.” And she is, but not in a good way. Exposed to a meteor just before the ceremony, Susan (voice of Reese Witherspoon) has a greenish glow. And then she starts to grow. Before she can say, “I do,” Susan is suddenly 50 feet tall. And before she can say, “How did I get to be 50 feet tall,” she is whisked away to a secret government compound for monsters where she quickly becomes a sort of house mother for a motley crew of assorted mutants, turning into a sort of cross between Alice in Wonderland in her giant mode, Snow White with the dwarfs, and Mary Ann with Gilligan, the Professor, and the gang.

Susan’s fellow monsters amusingly cover the full range of of B-movie monster origins. We have “The Fly”-style one mad scientist who became the victim of his own experiment with insects and turned into Dr. Cockroach, Phd (voice of “House’s” Hugh Laurie), one “Creature from the Black Lagoon”-style Missing Link thawed out of an arctic ice floe centuries after all others from his species had become extinct (voice of “Arrested Development’s” Will Arnett), and a giant bug (a la “Them” or “Mothra”). And then there is my favorite, Bicarbonate Ostylezene Benzoate, known as BOB (voice of Seth Rogan), a brainless but genial one-eyed gelatinous ooze along the lines of “The Blob.” These monsters are isolated as a matter of national security until an even bigger threat comes along. If you’ve heard the title, you know that it is aliens — or rather, one alien named Gallaxhar (voice of Rainn Wilson of “The Office”). He plans to take over earth. The monsters are the only hope of saving it.

It was filmed in digital 3D, in part an homage to the cheesy sci-fi films of the 50′s. It begins with the usual 3D trick as a bored technician plays paddleball and the ball on the elastic band seems to stop just short of our noses. But after that, the effects are more subtle and immersive. The animators have literally gone to unprecedented lengths — it almost feels as though we can touch objects that go back the length of a football field. The scenes are brilliantly designed to make the most of the 3D technology and the action scenes, particularly one on the Golden Gate Bridge, are as immediate and involving as any big summer explosion-fest. The story is fast-paced and funny, with many knowing references to classic sci-fi and a solid story of friendship and self-realization. The voices are all excellent, especially Stephen Colbert as the dim-witted President, Witherspoon’s Susan, who remains very real and human even after she becomes what the government christens Ginormica, and Laurie’s cockroach, who has the manners of a butler and the laugh of a mad scientist. And Wilson hits just the right note of petulance to keep the alien from being too menacing.

But the graphic character design is uneven. As with most animated films, the humans are often stiff and artificial. The big bug, the cockroach, and the Link are not particularly engaging. BOB, however, is simply sensational. Rogan’s husky voice and unabashed cheery laugh is a perfect match for the animated marvel of a big blue gooey thing that is endlessly pliant and effortlessly resilient. More than any other part of the movie, this charmingly silly little character shows what this technology is capable of, when the script has a great character to put on screen. In the battle between monsters and aliens, it is this little blue monster who saves the day.