Culture-consumed cats rehearsing their production after hours in the opera house may strike closer to the fancy of the adult reader than to the young listener's. However, in favor of Simon Boccanegia, it must be said that in spite of his mature liking for opera, he is a young-at-heart cat who appreciates a good can of tuna almost as much as an ear-ringing aria. Johnny Mulligan, Simon's master, Mr. Mulligan, the caretaker of the theater, and Mrs. Mulligan are among the few select humans invited to attend the cats' opera for cats -- presented on St. John's Eve when all cats can speak.