Check out "Supernatural" season 9, episode 13 spoilers and promo trailer video here! In next week's "Supernatural" episode, Sam and Dean suspect witches are sucking fat from day spa guests under the pretense of losing weight - it sounds like "Supernatural" season 9, episode 13 is going to be quite a strange one!

Watch the "Supernatural" season 9, episode 13, titled "The Purge," promo trailer video below! Check out what's in store when the episode airs next Tuesday.

Spoiler alert! According to TVGuide.com, the official synopsis for next week's episode reads: "Sam and Dean work undercover at a spa to investigate murders in which the victims were hundreds of pounds lighter after their deaths. All of the victims attended the spa for weight loss; and the brothers suspect that witches are to blame."

In the episode, the brothers go undercover, according to spoilers: Sam is hired as a fitness instructor, and Dean takes on kitchen duty. Dean steals some tainted food and passes out, and the boys soon realize that someone is drugging guests at the day spa and sucking all of the fat out of them until they die. Talk about an extreme weight loss method!

"Supernatural" season 9, episode 13 airs on Tuesday, February 4 at 9:00 PM EST on the CW.