 " Momentum Deferred " was written by screenwriters Zack Stentz and Ashley Edward Miller . In the first several drafts of the episode , it was initially called " This is the Night Mail , " which was based on a poem by W.H. Auden . The episode completed the " gun arc " storyline , which began with Olivia hurtling out of her SUV in the season premiere and being too weak to control her gun during her physical recovery , and ended with the killing of the shapeshifter posing as Charlie Francis . Miller and Stentz elaborated , " I think it 's Jeff Pinkner who described the first several episodes as the gun arc . Because really the gun played out until Olivia was whole again . Her actions with the gun in all of those episodes symbolized where she was on the journey . And it culminates in her being able to use it to kill someone who looks exactly like her best friend . " 
