Danny's (Elisabeth Moss) a new cop on the force, but the guys aren't taking well to it - it doesn't help that she's a woman in a man's world, and she's kind of a horror geek. She gets some ribbing from Marty (Stephen Lee) and Mattingley (Pablo Schreiber), and even sometimes her boss Sergeant Williams (Russell Hornsby in a pre-Grimm role!), but she's doing good work and sticking it out. A new prisoner comes through the doors on the snowy night Danny's supposed to stay late with Marty and Mattingley - he's an eater of human flesh and a magician, apparently, because he begins to eat the hearts of those around Danny and shapeshift into their bodies.
"Eater" works slowly, first developing the relationships between Danny and the other two cops she's staying with at the station before making anything else of the serial killer Mellor (Stephen R. Hart). Marty and Mattingley don't seem too sinister at first, but their pranks escalate from mockery to dangerous implications, like feeding Danny to Mellor or inviting her to check on him in the cell.
The police station is also very atmospheric; it's a claustrophobic place with only a few rooms, and as Danny walks back and forth between basement and jail cell we see the same areas over and over again, meaning that the audience can sense how trapped Danny is during the later chase scenes. And the lightly falling snow outside the glass windows helps to set the mood as well; there's not going to be an easy way to get away from the police station if Mellor attacks.
Stephen Hart is an imposing man, so Mellor's presence is frightening in itself; but the opening scenes detailing some of his grotesque work as a serial killer are disturbing enough to make me question how they got onto TV in the first place. Gordon does not cut out the gore, and color me impressed that "Eater" takes such a decidedly grim turn in its last arc.
The thing is, "Eater" never lets up. Its twists are surprising, and the finale is clever without losing its originality. The macabre outcome isn't a happy ending, but it at least stays true to the formula of the episode set up at the start. Gordon gives Fear Itself some gruesomeness that the show needed; it's too bad some of the other episodes didn't gorge on "Eater" and come up with a similar tone.