At the scene of a horrific car crash, the team finds three dead drivers and strangely, a human brain that belongs to none of the crash victims. Seeking to identify this "Brain Doe", the investigation leads them to the mysterious death and organ theft of a deceased serviceman whose brain was stolen from his body at a local mortuary by one of the car crash victims the night of the accident. Our CSIs discover the brain robber/crash victim, Clegg, swapped the brain found at the scene with the pathologist of an S.F.F fighter friend of his that appears to have just committed suicide. Our team follows the trail of evidence which leads to the fighter, Dempsey, and Clegg's former trainer, Pernin. Pernin was having an affair with Dempsey's wife and when he found out the fighter wanted his brain donated to science because he sought to prove he was suffering from a degenerative brain disease, CTE, Pernin hired a pathologist to remove the fighters brain and swap it with the brain Clegg stole from the mortuary. All of this was to hide the fact Pernin was supplying his fighters with an illegal performance enhancing drug to keep them fighting longer even after head trauma. The ultimate twist is when Brass finds Dempsey's wife didn't kill him but another family member did. We meet D.B's oldest son Charlie after he is suspended from a college basketball game and D.B. is concerned his son is starting to go in the wrong direction.