A meteor shower rockets through open space. Four glowing pods are seen hiding behind the meteor shower. Suddenly the meteor shower makes a hard right and heads towards a distant planet Earth.
In a Chicago suburb, Stuart Pearson (Kevin Nealon) and his wife Nina (Gillian Vigman) head a family that includes 7-year-old Hannah (Ashley Boettcher); 17-year-old sister Bethany (Ashley Tisdale), who's just returned from a secret outing with her boyfriend Ricky Dillman (Robert Hoffman), and 15-year-old techno-geek Tom (Carter Jenkins) whose high school grades are low. Deciding the family needs some good old-fashioned togetherness, Stuart takes them to a holiday home in the middle of nowhere. Joining them is Uncle Nate (Andy Richter), Nate's 14-year-old son Jake (Austin Butler), Nana Rose (Doris Roberts), and identical 12-year-old twins Art (Henri Young) and Lee (Regan Young). Ricky also arrives unexpectedly and talks his way into staying overnight.
That night dark storm clouds swirl around the house. Suddenly, the four glowing pods land on the roof. An alien crew emerges, made up of Skip (J. K. Simmons), the tough commander, Tazer (Thomas Haden Church), a muscle-bound dude armed to the teeth, Razor (Kari Wahlgren), a lethal female alien soldier, and Sparks (Josh Peck), a four-armed techie, who is the only non-threatening intruder. Since the aliens knocked over the satellite dish Ricky and Tom are sent to fix it. Ricky then reveals to Tom that he lied about his car breaking down, and his age; Ricky is actually in college and is five years older than Bethany. Ricky makes Tom fix the satellite dish by himself but it is beyond repair. Investigating further, Tom and Jake (who unexpectedly shows up on the roof) discover the aliens. Ricky is captured and implanted with a mind control device. The aliens, called "Zirkonians", lay claim to the planet. Like a puppet, Ricky grabs the boys – but Tom and Jake break free and get back in the house.
It isn't long before all five kids except Bethany have seen the aliens. Tom takes charge and they discover the mind control device doesn't work on children, giving them a fighting chance. They realize they have a responsibility to protect the adults by keeping the aliens' existence a secret. The kids create makeshift weapons, like a home made potato spud gun. They repel the first alien attack from the attic and obtain Ricky's remote control and turn him against the aliens.
The kids orchestrate a scheme to get the adults out of the house and then ambush the aliens as they try to reach the basement, causing the gentle tech Sparks to become separated. He meets Hannah and they become friends. Unlike his alien cohorts, Sparks hates battle; he just wants to return home to his family, and he helps by making weapons for them. He also reveals the aliens want a device hidden under the basement which will make them grow giant. The kids forgot about their grandmother, and the aliens mind control her, which gives her super-human strength and agility, and she fights with Ricky in a scene reminiscent of a fighting video game. However, the aliens manage to capture Jake and hold him hostage in exchange for Sparks, whom they need to complete their mission.
The kids finally reveal to Bethany what's going on. Ricky then breaks up with Bethany because she talks about feelings and family too much and leaves. The five of them rescue Jake, but Skip succeeds in using the enlarging machine and calls the Zirkonian invasion ships. They defeat him and shrink him again. Tazer and Razor flee, while Skip is shrunk to a tiny size by the machine. Sparks calls off the invasion and returns home. The rest of the vacation goes back to normal, except the kids grew closer to each other during their adventure. In the distance, Skip appears, bent on revenge, but meets his demise when a crow flies over and grabs him.
During the credits, Bethany and Tom take revenge on Ricky by making him look like a fool in front of his new girlfriend, using the alien mind control device. Bethany comments "I am so keeping this".
