Derek comes back to Beacon Hills for a number of reasons. The most important one, though, is the result of a one night stand in Vegas. He's knocked up and Alpha, and Cora won't stop playing Papa Don't Preach whenever she gets to pick the music in the car. This shit just keeps happening to him. Or, in which Derek gets the D, Stiles gets the D, then Derek gets Stiles's D.

It takes Stiles four days to notice something’s wrong. Well it’s not like jerking off three times a day is unusual behavior. The third day, he has his usual good morning session, comes straight home from Lacrosse and jerks off without even taking off his jeans because it’s laundry day tomorrow and it’s not like he’s got standards. And everyone jerks off last thing at night; it’s like a glass of warm milk for your dick. Stiles has been hit by a jerking off curse. He might not make it through May unscathed.