After Donnie was told about the end of the world a plane engine crashed through his house, destroying his bedroom. Luckily Donnie was outside and survived. Donnie also finds love in a new girl to his school named Gretchen Ross. This girl also has a dark past and is somewhat mentally unhealthy as well due to her mother being stabbed four times by her step father. As time goes on Donnie sees more of Frank and more hallucinations, which leads him to think that time travel is possible along with the power to manipulate fate. To prevent all of the negative events Donnie sees himself causing in the future, he goes back in time to stay in his room and die as the plane engine crashes through his home. This prevents a whole butterfly type effect on everyone that surrounds him and the end of the world Frank warns him about.
Throughout the movie Donnie is filled with unhappy thoughts and his head is consumed by troubled feelings caused by the prescription pills he takes, although these pills seem to be the only thing that Donnie has to give him a consistent behavior and focus in