Stiles gets friendly with a Tentacle Monster.

In which Derek finds Stiles in a compromising position.

Derek goes into heat. Stiles gives him what he needs.

Stiles has to perform some kind of protection spell/ritual to stop the latest threat to BH, but according to Deaton and his terrible, terrible magic books, it requires that he willingly take the ‘life essence’ of an alpha within him so he can power the spell. Yep, semen, because druids were giant pervs back in the day. And apparently that shit is like a magical battery.