The paper-thin plot can be summed up as follows: two alien kids, Sara and Seth (Anna Sophia Robb and Alexander Ludwig), crash land on Earth where they hire the services of a cab driver (Dwayne "the Rock" Johnson). Evil government agents and an even eviler cyborg space assassin chase them as they try to make their way back to the safety of their ship. Of course there are the expected themes of friendship and saving the environment, but the main point of the film seems to be for "future Disneyland theme park ride," as it features an inordinate number of car chases, including a "driving down the train tracks in a tunnel" scene that feels as if it was actually filmed in an amusement park.
Canny kids will question the need for all the running away, as late in the film it's demonstrated that the alien children have powers capable of negating any threats the human race can throw at them. Really canny kids will question the need for the Rock's role at all, as alien super-being Sara can use her powers to psychically take control of the car whenever she deems it necessary. Even the Rock is stripped of the charmingly humorous persona he's shown in previous films, though his work with Witch Mountain director Andy Fickman in The Game Plan was arguably his weakest performance.
Parents might be lulled into the belief that though they aren't enjoying the film their kids are having fun, but there is a fine line between sitting quietly in the theatre because you are engrossed in a story and sitting quietly in the theatre because you are nearly comatose with boredom. (Walt Disney)