It is difficult to tell if ”Revenge,” the fourth novel published by Stephen Fry, the British actor, is a daffy update of ”The Count of Monte Cristo” or a droll joke about it. Our hero, upstanding young Ned Maddstone, is in possession of a bright future, a lovey-dovey girlfriend…and half an ounce of hashish. Schoolmates jealous of ”floppy-haired, goody-two-shoed, squeaky-clean, doe-eyed, prefect-perfect” Ned planted the dope. That the cops also find evidence implicating the lad in an IRA plot is simply one ridiculous twist in a novel that constitutes a parade of them. Ned gets hush-hushed away to a psychiatric prison in Scandinavia. Of course, he escapes. Of course, he exacts justice. Of course, ”Revenge” is a preposterous melodrama, but–in its light, breezy way–a charming one.