The story itself is head-scratchingly absurd, like a beta version of “Forrest Gump.” Everything in it is funny or stupid or stupid-funny. Steve Martin’s “The Jerk” (1979) is so obliviously innocent, the edgier jokes and witty wordplay catch you with your guard down. “Did I just see that? Did he just say that?” Somehow, the rags-to-riches-to-rags story of a man who seemingly floats from one unrelated set of hijinks to another (gas station attendant, carnival barker, millionaire inventor) is able to get itself together by the climax in a way that many modern comedies cannot, while still delivering continuous gags.