In the chaos that erupted around Agatha's escape from Castle Wulfenbach, several of the students took the opportunity to get out themselves. Specifically, Theo, Sleipnir and Z slipped out in the confusion. They might have taken Zulenna, but she had been killed during the mess, and her only hope of survival apparently lay with Klaus. So they went off on their own.
Von Pinn has, of course, been tasked to recapture the runaways, and bring them back under her own loving ministrations. She's been working on it for months now, but without success: they've managed to keep a low enough profile that she simply doesn't know where they are. But there is something (need to figure out what) about the Hidden Castle that has specific meaning for one of the runaways -- probably Sleipnir, but that needs more fleshing out. Von Pinn knows them well enough to know that it might attract them, so she is along on the trip, to recapture them.
That said, the situation is messy. Klaus' priorities on the subject are less strict: he wants them under his control as good subjects and hostages; having them on board the castle is a secondary priority. And Gil has a good deal of sympathy for them: he'd love to be able to escape his own responsibilities. So it's far from clear how this will come out.
Finally, there is Zulenna. While there wasn't much that they could do for her, she's still feeling rather abandoned and lonely. Her plot and theirs should be tightly intertwined.
Justin 1/1: Let's take this plot and seriously rework it. Currently, both Zulenna and Von Pinn are rather isolated: they're not well-tied into any factions. So instead, let's say that only Theo is the runaway. What happens?
High concept: Theo escaped in the confusion of Agatha's escape, but the others didn't. Z felt that it would be dishonorable to leave Zulenna alone in her slightly dead condition -- she would want friends around when she was revived, and they were the closest thing she had to them. And Sleipnir simply got caught in the course of the escape.
Since then, Von Pinn has been watching them like a hawk, which hasn't been fun for anyone. The three of them wound up getting closer, although Zulenna remains prickly. And so Z and Sleipnir insisted, very firmly, on accompanying her when she was forced to come home as part of the Succession plot. Klaus agreed, but only if Von Pinn came with and chaperoned them (and made sure they came back again) -- he feels it is useful experience for all of them to see how the world really works.
So the three of them have wound up tagging along in Zulenna's wake as she deals with the Succession plot, and they each have different priorities. Von Pinn wants to keep them safe, but also under control. Zulenna wants to figure out the succession. Z has developed feelings for Zulenna, although perhaps more a strong friendship than anything else. And Sleipnir wants out -- specifically, she wants to reunite with Theo, and get out from under the Baron's thumb.
Note that this plot is closely related to Suspected Revenant.
Characters In Game: Gil Sleipnir Theo Von Pinn Z Zulenna
All Characters: Gil Sleipnir Theo Von Pinn Z Zulenna