Frankly, it's hard to talk about this movie without giving some of the insane plot points away. But here goes. It starts with Samuel L. Jackson narrating in a semi-Tarantino homage. That, however, turns out to be a fake out. Instead, it's narrated by an unidentified female voice that turns out to be related to the big reveal at the end. The Sam Jackson bit is actually a screenplay that Oscar Isaac's Will is trying to write. Will is in a state of deep depression because his wife, Abby, left him. Or did she leave him? Or did something horrible happen to her? Take one guess.
Will tells the story of their romance to his therapist (Annette Bening). They met in college, and he was obsessed with her to a degree that is pegged as being romantic but actually just ends up being incredibly creepy. The movie briefly acknowledges this, saying that he loved her "with an intensity usually reserved for stalkers," but basically brushes that away. He's Oscar Isaac! He's so handsome! She's a little weirded out by it, but loves him too because they both like Pulp Fiction, so it's all OK.