Sam and Dean discover a doctor who, back in 1816, became obsessed with finding the secret of eternal life by taking people's vital organs. Meanwhile, Bobby finds Bela and Dean leaves Sam to confront her.

Hunter Rufus Turner calls in Bobby, Sam, and Dean to deal with a demon infestation in his town, but they soon discover the truth is much more horrifying. Meanwhile, Castiel goes to seek the only entity that can defeat Lucifer.

After Bobby discovers Crowley has no intentions of returning Bobby's soul, the hunter takes matters into his own hands. He calls on Sam and Dean for help after he unearths one of Crowley's deepest secrets. Meanwhile, Sam and Dean learn what Bobby does all day when they are not around.

Eve, the Mother of All, unleashes a new monster upon the world: a black worm-slug that can enter the brain and force people to kill. Sam and Dean join forces with Bobby and Rufus and track it to a cannery, only to discover that Gwen and Samuel are also there looking for the monster... and none of them know who the monster is inside.

As Sam and Dean race the clock on a high-stakes mission, Bobby seeks the help of an unlikely old friend to solve one of the most personal cases of his life.

A dangerous creature is accidentally released into an old house, attacking a mother and her child, leaving them both in a coma. Sam and Dean learn that Bobby and Rufus once tracked the same entity so the Winchesters look to the past to come up with a plan to catch the monster before any one dies.