Castle - Season 4 Episode 22
Undead Again

When Castle and Beckett investigate the murder of a man with human bite marks on his body, Castles wild theories start flying. But none are as wild as what their only witness insists happened - a Zombie attack. As the evidence pointing towards an undead assailant piles up, the team plunges into New Yorks Zombie subculture to find the killer and bring him in -- dead, undead or alive