Thallium.

Nicole accepts a mysterious invitation and, to her surprise, meets her old circle of friends. The former friends, who haven't been in touch for years, come together for a reunion at an idyllic holiday cottage. When the friends were at university, they played a trick on some fellow students in their dorm, which ended in tragedy. Two people died. The police couldn't prove anything. Now they are called out to account for their actions in a macabre way.