From the liner notes

They say vacations never go according to plan, and school trips are no exception. Instead of enjoying sunny skies in Okinawa, Sousuke and Kaname's classmates are under cloud cover in Siberia, playing the part of hostages. A vicious and unbalanced terrorist has kidnapped Kaname to donate her brain to science-with or without her consent! Will Sousuke and MITHRIL be able to scramble an ad-hoc rescue? Will they be able to get everyone out alive?

Contains episodes 5-8