Robert Downey Jr. plays Peter Highman, a first-time father whose wife's due date is five days away. As Peter hurries to catch a flight to be at her side, his best intentions go awry when a chance encounter with aspiring actor Ethan Tremblay (Zach Galifianakis) forces Peter to hitch a ride with the slacker—on a road trip that will ultimately destroy cars, friendships and Peter's last nerve.