When Andrew Crompton, amateur astronomer and Master of Gresham College, is found dead at the foot of the University Observatory stairs, Lewis and Hathaway find that the finger of suspicion points at all the staff - from tutors right down to the head porter and the college scouts. Lewis uncovers a college-wide blackmail plot and is confused by a mysterious astronomical conundrum. Hobson is unexpectedly drawn into the case while rehearsing for a performance with a local orchestra at the college.