At his residence in Beckenham in Kent on the outskirts of London, Sir Andrew Wiggenhall the chairman of the board of directors of a large firm of ironmongers is killed by an explosion that largely wrecks the house. Investigating officer Superintendent Hanslet is not convinced his death was an accident, particularly when another member of the firm's board is found dead. With the help of Priestley he sets out to solve the mysterious incidents that have left the remaining members of the board.