In “Magnum Opus,” an Oxford college dean is found bludgeoned to death in the woods following a heated debate in a pub. An alchemic image purposefully left at the crime scene leads Lewis and Hathaway to anticipate that three more murders will follow. An unusual tattoo on the dean’s body and the bodies of two more victims leads to the discovery of a secret esoteric society and a frantic effort to prevent a fourth murder. Meanwhile, Hathaway is struggling to accept his father’s illness.