Or, Harry wakes up in a mental institution, where he's spent the last dozen years as a patient. He's informed that a serial prankster posing as a mailman was lacing people's mail with LSD. Harry had a particularly bad reaction to it, resulting in brain damage that caused hallucinations.
Additionally, his doctors inform him that his refusal to analyze aspects of his past within the hallucination was his mind's way of avoiding memories that would conflict with his fabricated reality.
Upon his return to the real world, he meets his fellow asylum patients, each of whom he's cast as different allies or enemies in his delusion. To help him accept that his memories are false, he's shown security footage of him acting out his fantasy in the asylum's common area, where others joined him in his frantic behavior.
Ultimately, his "case files" are actually the detailed patient files logged by Dr. James Butcher, who has been working with him to try and break through the delusion.
