Full Metal Panic! Volume 2

Two months after Sunan, Sousuke and Kaname are in a holding pattern.
He'll fire his rifle off the school roof. She'll yell at him to stop. Lather, rinse, repeat.
That all changes one day when Sousuke's commanding officer Teletha "Tessa" Testarossa shows up at his apartment, trying to hide a key to a terrorist superweapon of unprecedented power. Tessa is brilliant, pretty, and the same age as Sousuke, and her presence might put a serious crimp on his relationship with Kaname... assuming the city survives the night!