Las Vegas cabbie Jack Bruno (
Dwayne Johnson) finds himself stuck with two teenaged siblings—Sara (
AnnaSophia Robb) and Seth (
Alexander Ludwig)—who claim to be aliens running from the "Siphon" (
Tom Woodruff Jr.), an assassin from outer space sent to kill them. They are also being pursued by Major Henry Burke (
Ciarán Hinds) of the U.S. Defense Department. The only way Jack can think of aiding the two aliens is to ask for help from Dr Alex Friedman (
Carla Gugino), a scientist who is presenting a lecture to a UFO Convention at Planet Hollywood. Together, the four of them race to Witch Mountain where their spacecraft is secretly parked.