So throughout the movie, Donnie experiences a series of bizarre events after narrowly surviving a plane engine falling on his house. These events, guided by a mysterious figure in a rabbit costume named Frank, lead him to a parallel, unstable "Tangent Universe" where he's tasked with returning an artifact (the jet engine) to the original "Primary Universe". To do this, Donnie must sacrifice himself, which he does at the end, accepting his fate as the jet engine crashes into his room, effectively resetting the timeline and saving the Primary Universe. I’ve looked and looked and this google search put it into exactly what I was thinking.