Larry Daley is a Brooklyn divorcee whose unstable work history causes his ex-wife Erica to believe that he is a bad example to their ten-year-old son Nick, and Larry fears that Nick respects his future stepfather Don more than him. Cecil Fredricks, an elderly security guard about to retire from the American Museum of Natural History, hires Larry despite his unpromising résumé. The museum, which is rapidly losing money, plans to replace Cecil and his two colleagues Gus and Reginald with one guard. Cecil gives Larry an instruction booklet on how to handle museum security, advises Larry to leave some lights on that night, and warns him not to let anything "in... or out".