When a commercial shoot goes terribly wrong, Rex, Hollywood's biggest canine star, goes missing and is presumed dead by his handlers. Rex takes refuge in a row of lofts, but the structure catches fire, and Rex makes a death-defying leap into a fire captain's (Bruce Greenwood) arms. The man's son, Shane (Josh Hutcherson), becomes Rex's reluctant new master, and the firefighters adopt the dog as their mascot. Trouble looms when Rex's handlers try to get him back.