Complex sentence: Dexter invents a shrink/grow ray, and aims to shrink Mandark's head; but Computress accidentally throws the switch to grow, and Mandark's head expands to the size of the universe, and then bursts into millions of tiny heads.
Split into simpler sentences: Dexter invents a shrink/grow ray, and aims to shrink Mandark's head, but Computress accidentally throws the switch to grow. This causes Mandark's head to expand to the size of the universe and burst into millions of tiny heads.