William Zero’s big twist is that the “original” William isn’t original after all. He was the first clone, created by the actual prime, who then abandoned him. Moreover, the first clone is evil, and has created Proxy William to serve two functions: to handle the day-to-day business of being William so that the first clone can pursue a life of amorality and murder, and to win back Jules so that she’ll trust the first clone, who then plans to kill her to spite his creator. But Proxy William fights off his maker, and then meets the real original, who comes home to find “himself” dead on the floor and a blissfully ignorant Jules in his bed. The proxy then gives the prime the gift of starting over, letting the world believe he’s dead while he escapes with Jules and uses his technology to clone the son that he accidentally killed at the start of the film.
In a different kind of movie, that might’ve been a clever bit of plotting. Here it results in the last third of the film mixing overheated stand-offs and long conversations about who’s who and the nature of good and evil. The twist shifts William Zero’s gears from meditative and brainy to pure potboiler.