Peter Highman (Robert Downey) is an expectant first-time father whose wife is due to give birth in five days. Peter intends to catch a flight home from Atlanta so that he can be there for the delivery, but a chance encounter with aspiring actor Ethan Tremblay (Zach Galifianakis) throws a monkey wrench into his plans. Desperate to reach his wife's bedside in time, Peter's sanity hangs in the balance when he is forced to take a cross-country road trip with dog-toting oddball Ethan.