Things picked up right where they were left after "Formality."  The Alpha is trying to gather Scott and Derek, but the latter is still captured by Kate.  Stiles is forced into helping the Alpha, but Scott finds Derek first.  The two of them escape only to get hunted down by Kate and Allison.

Just before Kate kills Derek and Scott, Chris shows up and stops her.  Chris explained that they hunt by a code, and that the people who died in the Hale fire were innocent.  The Alpha then takes Kate away and kills her.  He goes after Allison, but Derek and Scott save her.  

Jackson and Stiles show up, and everyone teams up to take down the Alpha.  Instead of letting Scott kill the Alpha to cure himself, Derek murders his uncle and becomes the alpha himself.  After asking for it again, Derek bites Jackson.