Something like a love story ensues, and while “Looper” briefly ponders whether young Joe’s new feelings for Sara will erase Old Joe’s memories of his wife, the butterfly-effect games largely slow to a halt. From a series of baroque curlicues, the movie straightens into a line, and the heady inventiveness of the early scenes dissipates. The final twist is both logical — inasmuch as anything in this movie is logical — and disappointingly abrupt, as though another shoe were hanging up there refusing to drop. With “Looper,” Johnson proves he can finesse the most complicated notions and visual setups his mind can imagine. It’s the simple things that still elude him.