4x10 - Blue Murder

When math professor Nathan Cuthbert is found dead, Oosterhuis and Stahl investigate, and Oosterhuis is soon warned off the case by Colin Gladden, his former partner when he first started with homicide. Unfortunately, Oosterhuis disappears during the investigation. Bender and Weeks investigate the suspicious death of journalist René Cloutier, who was found hanging in a closet wearing women's clothing. While investigating Cloutier's death, which turns out to be the result of poisoning, Lukowich tells Bender and Weeks that Cloutier was killed by the same poisoned drug as Stahl and Oosterhuis' victim.