A bunch of unnamed witches (one of whom eventually introduces herself as ""Wanda"") stage an elaborate Halloween amusement, somewhere out in the woods, for a bunch of unnamed kids who somehow turn up. The text is all in dialogue--snatches of dialogue unidentified with any particular speaker (the witches talk about what they'll do, the kids voice their reactions to what they fred); the dialogue is studded with italicized words (witch: ""Wild idea! I love it!"") and dispensable observations (""Nothing is too good for children""); the only semblance of a plot-episode is the children's ride, at the close, on bat-wing gliders--the surprise ""main event"" that Wanda has alluded to earlier. In its amorphousness the whole thing does somewhat resemble the ""happening"" of the title (it also might be dubbed Halloween-World); but even Adams' full-color pictures of the witches' skyscraping tree-and-pumpkin fun house can't make it into an involving book experience.