Liza De Weerd

Joe Carroll puts his final plan into action, threatening the lives of many, as Ryan and Mike go to great lengths to stop him and prevent more innocent lives being lost. Meanwhile, Claire makes a decision that leads her into dangerous territory, and Mark and Luke attempt to figure out their next move.

Sydney becomes infected with a mind-altering and potentially deadly hallucinogen, causing her to slowly lose touch with reality. Vaughn and Jack must scramble to find an antidote before Sydney's hallucinations, paranoia and resultant violent behavior threaten her life and the lives of those around her.

Monk reluctantly adopts a dog while looking into the disappearance of its owner.

"Grey's Anatomy's" three-episode story arc continues with a part two that will challenge the interns of Seattle Grace -- and "Grey's" fans as well -- like never before.