Arthur is a comedy which relies totally on the terrible jokes of an mostly unlikeable lead who acts inebriated for the whole film. At some point in the movie you realise that you're laughing at an alcoholic, or at least that you're not laughing at an alcoholic and this is before realising that the premise of the movie is ridiculous to the point of being unbelievable.
Arthur, a bored and spoilt multi-millionaire and alcoholic, prides himself on being funny and rich, his inheritance is threatened when he refuses to not marry the suitor his parents choose for him. Marriage, in the eyes of her parents is Arthurs right of passage into adulthood, however his wry, snobbish but loving butler and surrogate father correctly surmises that love will be the catalyst for Arthur's growth.
It is the butler who provides the only pathos and emotional interest in the film, and thankfully some comedy with his sarcasm and casual prejudice toward everything and everyone. After he exits the picture in a way that tonally is not handled well at all, and in fact could go unnoticed if you blink, we are left with the a long wedding scene which is basically and literally the punchline to a joke that was never funny, and is actually quite distasteful.
Maybe alcoholism was inherently funny in a bygone era, funny enough maybe to carry a movie with few jokes, a ridiculous plot and very little charm. Unfortunately for Arthur, he has not aged like a good Scotch, and instead a bitter taste is left in the mouth after watching this.