Supernatural (TV series)/Recap/S06/E08 All Dogs Go to Heaven

Season 6, Episode 8:

All Dogs Go To Heaven
Sam and Dean discover the Alpha skinwalker's plan: create sleeper cells of his kind and turn them loose upon humanity. To find and stop the Alpha, the brothers need the help of a skinwalker that has taken the form of a dog and befriended its human family.

Tropes
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 * Voluntary Shapeshifting -- the skinwalkers