Complex sentence: Bart destroys some of the ecosystem in Lisa's tub universe and the people respond, sending a squadron of space ships to attack Bart, who vows revenge on the small universe and Lisa wonders what to do.
Split into simpler sentences: Bart destroys some of the ecosystem in Lisa's tub universe and the people respond, sending a squadron of space ships to attack Bart. Bart vows revenge on the small universe and Lisa wonders what to do.