For all their insistence that they can control their shift (just as well as Isaac now can), they sure ran to Derek fast when they realized how wrong they were.
Derek just about thanked every deity he could think of for the timing of all this. If this had happened any sooner, Scott wouldn’t have trusted him, wouldn’t be in his pack right now. And, considering how strong those two rebellious pups are, Derek’s going to need all the help he can get to keep them from succumbing to Peter’s control.
With Scott comes his best friend Stiles; there’s never one without the other. And, usually, Derek would tell Stiles to get lost and stop being so careless with his human life, but right now, Derek is happy to have him aboard.
While Stiles and Isaac take charge of keeping Boyd from running to the Alpha, Scott and Derek handle Erica. It goes well for a bit, until it doesn’t. Until Isaac is knocked out, and Stiles is only human, so Boyd escapes. Luckily, Peter is busy capturing and torturing the Kanima’s controller when that happens, so Derek finds Boyd and drags him back.
Erica is a handful. She’s ten handfuls. Sometimes it takes Allison shooting her with arrows to keep her away from any and all exits. But when Erica finally gains control, she’s nearly as good as Scott.
The problem with focusing on specific people is, often, one loses track of the people they aren’t paying attention to.
Stiles is the first to notice that Isaac is missing. And by the time Derek returns with Scott from training Erica out in the woods, they track the scent all the way back to Peter, Lydia and Jackson.
Derek says they can’t get him back now; that they aren’t ready yet. Most of them drop it because they know it’s true. But Stiles can’t. If they leave Isaac – one of their most reliable pack members – with Peter, then it’s just a matter of time before Peter wrangles another of them – a weaker member.
No-one listens to Stiles, though. Even Scott takes Derek’s side.
And then Stiles is gone soon enough. They hadn’t considered Peter would go after the humans when there are Betas waiting, ripe for the picking. Scott, surprisingly, isn’t the one who wants to plan an attack on Peter. It’s Erica.
Allison agrees that they need to get to Stiles before Peter turns him, and has two Betas bonded to him through that. Derek says it’s probably too late; they need to concentrate on making their pack as strong as possible first.
It continues like this. One by one they disappear. And each time, Scott tracks the scent back to Peter. Eventually, it’s just Scott and Derek. Allison had been hidden in one of her family’s storage places. At least she would be safe – for a while.
When Chris tells Scott that Allison has been taken, he can’t ignore it anymore. He goes off, on his own, to confront Peter. Derek returns to an empty home, leaving him powerless and utterly alone.
This must have been Peter’s plan all along. To take everyone who’s important in Derek’s life away from him until he could either fight an Alpha by himself (and die) or join Peter’s side. Derek refuses to consider either scenario, but goes to Peter nonetheless.
What he finds when he arrives is something he thought he’d never see.
Derek’s pack is there, yes, but other people have been turned as well. Stiles’s father – to ensure Stiles would never leave - Scott’s mother, Allison’s father, and so on. If they left Peter’s side, they would have had no choice but to leave their blood relatives behind as well. Derek knows how difficult that is, especially for one with werewolf senses.
Derek’s entire family – blood is of no importance – is standing before him, waiting to see what he decides, but Peter knows. Peter always knows.
“I’m glad you finally decided to join us, Derek,” he says calmly, clapping Derek on the shoulder. “Now we are complete.”
Derek says nothing, which is all the answer Peter needs.