A wealthy financier is murdered in his desirable mansion.Two lieutenants,a tough guy and a rookie,investigate.After questioning the servants and his partner in an African cotton plant,they learn that the victim's mistress has escaped to Marseille to meet a young man.Although she claims she's just broken with her lover,she's the main suspect: charged with murder, she's imprisoned.But the young detective is not convinced:a button was found near the dead body,and this may be a vital clue.