Complex sentence: Special Agent Frank Lundy returns to hunt down the Trinity Killer and begins working with Debra to hunt down Trinity, leading to Debra cheating on Anton with Lundy and shortly after, an unseen assailant shoots them both, killing Lundy.
Split into simpler sentences: Special Agent Frank Lundy returns to hunt down the Trinity Killer and begins working with Debra on the case. This leads to Debra cheating on Anton with Lundy; shortly after an unseen assailant shoots them both, killing Lundy.