When some skeletal remains are found inside a bricked up wall of a canal tunnel, Barnaby takes on the challenge of finding the cause of death for one of them who was killed decades after the others (who presumably died in a cave-in.) Meanwhile Troy investigates his last case in the area -- the killings of young men who were known to be harassing a local homeless man -- and dealing with the resentment of his former co-workers now that he has been promoted to Inspector.