An Unstable Quadrilateral

An ordinary guesthouse, four wealthy couples, and two students on vacation. A torrential downpour brings all these unrelated people together. As people die one after another and the truth is gradually uncovered, the clues lead to an old case from eighteen years ago. A revenge plot unfolds among them. Can Chen Hai unravel the mystery and stop the killer before another victim appears? (A mind-bending mystery, not for readers who prefer lighthearted whodunits.)