S04E14: 'Sex And Violence'

The Winchesters arrive in Iowa to investigate why three seemingly happy men have all murdered their wives within the space of two months.

Enlisting Bobby's help, the brothers determine that the husbands have been put under a "love spell" by a mythical creature that has the ability to entice men to do what she wants.

Craving love and loyalty, a Siren will use her ability to read minds and transform her appearance to tempt men to prove their devotion by destroying the people closest to them in her honour.

According to lore, the only way to kill a Siren is by giving her a taste of her own medicine, but the boys' partnership hits a stumbling block when the Siren manages to weave her spell on both of them, prompting the siblings to voice their true opinions of each other.