Why does Ben Stiller infiltrate the Smithsonian institute....

I thought he was just there because he was down on his luck and accepted a job as night watchman at the museum.  Here is a synposis from amazon.com:

An irresistible concept meets computer-generated wonders in Night at the Museum, inspired by a 1993 children's book by Milan Trenc. Ben Stiller stars as Larry Daley, an underachieving inventor waiting for his ship to come in while getting evicted from one apartment after another for lack of funds. Larry's son needs some stability, so the well-meaning ne'er-do-well takes a job as night watchman at New York City's Museum of Natural History. What the soon-to-retire guards (Dick Van Dyke, Mickey Rooney, Bill Cobbs) don't tell him is that an ancient pharaoh's tablet in the museum causes everything on display to come to life at night. Thus, Larry meets representations of Teddy Roosevelt, Attila the Hun, fire-worshipping cavemen, and Roman Empire soldiers, and learns to cope with an excitable T-Rex and man-eating, ancient animals. The film might have left things at that, but an added story element gives Night at the Museum some extra urgency and excitement, especially for kids: Larry becomes responsible for keeping this nightly miracle going and preventing anything in the museum from dying due to exposure to sunrise. Computer effects, as well as wildly imaginative costumes and makeup, help make the film appeal to the 8-year-old in everyone. Director Shawn Levy (The Pink Panther) works with a hugely talented cast, including Robin Williams, Owen Wilson, Ricky Gervais, Carla Gugino, and Steve Coogan. --Tom Keogh