Complex sentence: Just then, Paul's phone goes off, and when he learns why he is needed he reluctantly heads over to downtown Metropolis to join the police force in confronting Doomsday while Superman's still fighting it.
Split into simpler sentences: Just then, Paul's phone goes off, and when he learns why he is needed. He reluctantly heads over to downtown Metropolis to join the police force in confronting Doomsday while Superman's still fighting it.