Complex sentence: Butch impresses Tom's girlfriend with more weightlifting, but Tom, determined to win his girl back, interrupts and impresses his girlfriend with his own displays, starting games of one-upmanship between Butch and Tom over the heart of the girl cat.
Split into simpler sentences: Butch impresses Tom's girlfriend with more weightlifting, but Tom, determined to win his girl back, interrupts. Tom impresses his girlfriend with his own displays, starting games of one-upmanship between Butch and Tom over the heart of the girl cat.