"But Why Bump Off Barnaby?"

When Barnaby is murdered at a family gathering, it transpires that he had a motive to kill everybody else but no one had a reason to want him dead. While dying, he left a clue which can implicate anyone. While the bizarre group frantically tries to unmask the murderer, people vanish, drinks are poisoned and the police take forever to arrive. Meanwhile, there's a secret treasure to be found, a mystifying limerick to decode and all sorts of doom to be avoided before the killer is finally unmasked.