Complex sentence: The story is told in a nonlinear order and events become clear through flashbacks (or time travel experiences) from the unreliable narrator who describes the stories of Billy Pilgrim, who believes himself to have been in an alien zoo and to have experienced time travel.
Split into simpler sentences: The story is told in a nonlinear order, and events become clear through flashbacks (or time travel experiences) from the unreliable narrator. He describes the stories of Billy Pilgrim, who believes he was held in an alien zoo and has experienced time travel.