London, 1924. After the dramatic events of 'Four Days in October' and the exciting General Election, the Three Just Men and One Woman intend to return to solving some of the other criminal cases crying out for their attention. Mirabelle and Leon have an Italian banking case to investigate which would take them away from England and the influence of the irrational and disruptive Maria of Gratz. It's time to return to the sort of cases that made them famous! But Europe in 1924 is a dangerous place; perhaps they should stay in London and investigate a missing jewel -- a valuable sapphire brooch that was given away to a notorious criminal. But as it happens, Mirabelle and Leon know the criminal ... 'The Sapphire' continues the story of 'Four Days in October' and 'The Woman of Gratz'. It is based on the characters in Edgar Wallace's thriller 'The Three Just Men' (1924)