Complex sentence: Bart is the only other one to escape the poisoning, having fed his oysters to Martin's cat; he stays behind and spots Principal Skinner and Mrs. Krabappel kissing inside Martin's playhouse.
Split into simpler sentences: Bart is the only one to escape the poisoning, having fed his oysters to Martin's cat. Bart stays behind and spots Principal Skinner and Mrs. Krabappel kissing inside Martin's playhouse.