A middle-aged man is being chased through a residential area by four men. He's finally caught and pummeled by those guys. When Frank, Alex and Horatio get to the scene, it's discovered that the guy was a sex offender and that, even though it looked like it, he wasn't beaten to death. A stab mark on the side of torso shows he was stabbed before he ran and was beaten up. The CSI team has to find the killer.
That trails leads, as it usually does in this show, to a number of suspects....in this case, since molesters are so reviled, there is a large number of people who might have done it.
A sidebar story involves Delko and his love-life, and if it went dangerously too far.