Complex sentence: A man named Howard explains that he saved her life because there has been a massive nuclear or chemical attack, possibly by Martians, which has left everyone dead, and that she can't leave because the nuclear/chemical fallout contaminated the air.
Split into simpler sentences: A man named Howard explains that he saved her life because there has been a massive nuclear or chemical attack, possibly by Martians, which has left everyone dead. She can't leave because the nuclear/chemical fallout contaminated the air.