Hotch meets a woman named Beth in the park when they are both training to participate in a triathlon. Rossi hopes Hotch will follow through with the obvious attraction he feels for her. The team is surprised and hopeful when they find out that Hotch might be dating again after Haley's tragic death.
Two men are found beaten to death in an unusually savage attack, so the BAU heads to Philadelphia to investigate the murders. Garcia is convinced the level of violence the unsub has unleashed on his victims means more murders will occur.
The unsub is Jimmy, a boxer on the skids who is struggling to win some matches to get money for his terminally ill son, Ryan. Ryan has leukemia and is in dire need of a bone marrow transplant. Jimmy tells his trainer Tony that MMA fighters get much more money than boxers, and maybe he should steer towards that. Tony reminds him that he needs a lot more training before he's skilled enough to go into MMA.
Morgan and JJ are at the crime scene, and they note the unsub seems very organized in his attacks. He might have come from the hospital, since that's the only place open at the time of the attacks. The second victim was just trying to help stop the attack, so the unsub didn't specifically target the first victim. He just needed someone to kill. The second victim was also hit much more than the first one. It seems like the unsub has developed bloodlust, and will seek out another victim to attack just for the thrill of killing.
Jimmy gives his trainer money to bet on him for his next match. Reid believes the level of violence the unsub is enacting will cause seizures and hallucinations. This happens to Jimmy, and Tony suggests pulling out of the fight because he's not at his best. Jimmy refuses and steps into the ring. He doesn't seem to be doing very well until he sees blood seeping from his opponent's nose and imagines he sees Ryan in the crowd cheering him on. Jimmy is driven to a violent victory against his opponent. He is thrilled to get the money he won from the bet until Tony reveals he had bet against his own boxer. Jimmy kills the bookie and his bodyguard and takes the money to Ryan's doctor.
Dr. Surna says that Ryan needs chemo before he's strong enough for the transplant, but the boy isn't responding well to the chemo. Meanwhile the team figures out that one of Tony's fighters is the unsub, and Jimmy has made Tony his next victim.
Jimmy puts himself in for an MMA match, but he does badly, allowing himself to be injured severely enough to be taken to the hospital so that he can see Ryan. The doctor and Hotch tell Jimmy that Ryan doesn't have a lot of time left, and Jimmy is able to say goodbye to his son before he dies.