Complex sentence: Garfield tries to warn Jon only to be put into a trance when Mona Lisa scratches her back, & when she breaks into a sneeze fit, she remembers that she's allergic to cats.
Split into simpler sentences: Garfield tries to warn Jon only to be put into a trance when Mona scratches his back. Suddenly, Mona breaks into a sneezing fit and remembers that she's allergic to cats.