Donnie Darko is a quality movie. The idea of having a tangent universe is a hard concept to grasp but to make a whole movie would be difficult. Adding a Fourth Dimension when Frank can travel in time adds some spice to the movie. Frank being the manipulated dead allows him to travel back into time to save Donnie. When the jet engine falls through the portal between dimensions Frank warns Donnie to get out. Frank continues to help Donnie survive until the portal closes. When the portal closes the cycle is complete and Donnie remembers all of the future and dies in order to save the universe.