15-year-old Tom Pearson (CARTER JENKINS) isn't happy about going on vacation with his family. After all, his parents -- Stuart (KEVIN NEALON) and Nina (GILLIAN VIGMAN) -- have just caught him changing his grades online (they're mediocre due to him playing dumb to avoid being picked on as a brainiac), but they failed to notice that his older teen sister, Bethany (ASHLEY TISDALE), has just snuck in the window after a secret date with her smooth operator boyfriend, Ricky (ROBERT HOFFMAN). And then there's the fact that Tom simply isn't as cute and adorable as his 7-year-old sister, Hannah (ASHLEY BOETTCHER).
Even so, Stuart thinks a family outing to a remote, but large vacation home is just what the doctor ordered. So the Pearsons travel there and are met by Tom's mother, Nana Rose (DORIS ROBERTS), his brother Nate (ANDY RICHTER), and his sons, teenager Jake (AUSTIN BUTLER) and 12-year-old twins Art (HENRI YOUNG) and Lee (REGAN YOUNG). That's followed by the surprise appearance of Ricky whose family has a place close by, but he feigns car problems in order to be invited to stay overnight.
But he isn't the only unexpected visitor as four small but resourceful extraterrestrials also arrive there, ending up in the attic during a bad storm. Led by Skip (voice of J.K. SIMMONS), their recon team consists of Tazer (voice of THOMAS HADEN CHURCH), Razor (voice of KARI WAHLGREN) and newcomer Sparks (voice of JOSH PECK), and they need to get into the basement to retrieve something important regarding their intergalactic mission.
The kids stumble upon them and realize they're in a heap of trouble, but after their call to Sheriff Armstrong (TIM MEADOWS) is cut short, they quickly realize they can't tell their parents or grandmother since the aliens have devices that only seem to work on adults but turn such humans into remote-controlled zombies.
With the rest of the invasion pending, the kids do what they can to keep the aliens away from the adults as well as out of the basement.