The episode opened with the police receiving a call from Doc Robbins wife Judy that she had a dead man in her bed. CSI goes out and it appeared that Judy was having an affair and the man died of a heart attack.
As the case unfolds Brass refuses to believe that it could be anything other than an affair. After it made clear that there was another person in the house, the team got into high gear trying to determine who it was.
The DNA sample they found at the house was a family match to a man in jail. DB interviewed the man and he claimed there were no other male members of his family. However, with the help of a Genealogist Greg figured out that the man that killed the victim was the son and brother to the man in jail.
The killer was horrified when he found out that his mother had sex with her son (his father/brother) and then abandoned him at a church so he killed the genealogist (the victim) and tried to make it look like a sex thing, framing Judy.