Wendy and her friends, Mark, Seulgi and her twin Wonwoo, are headed to the parking lot when they witnessed the explosion and burning of one of their university's science department. Fearing that there may be other students or professors trapped in the blazing building, they rush to the rescue - and unexpectedly find themselves with a mystery that seems to be insoluble.        The first clue is an anonymous black box, its contents are pictures of students and one very familiar face. Even more puzzling are the faces printed on the papers. They were the students who mysteriously disappeared.        Who left the box? Is he responsible for the explosion? If so, why did he do it? And does he know something about the missing students?         Fire investigators, together with the police, believe that the destructive explosion and fire may not have been an accident - but an act of revenge. Evidence mounts against Steve Montgomery, a former professor whose daughter died in a tragic accident the night of the university's field party. Will the young sleuths be able to solve this mystery?