The skilled burglar Fini was unable to open the locks of the mysterious door and was killed. His body is discovered in his apartment by Scotland Yard inspector Dick Martin. Soon the inspector resigns and, on behalf of the guardian of the young Lord Selford, follows in the footsteps of the lord traveling around the world. On the way, Martin meets a young relative of Selford, who inherited an ancient key. When it turns out that someone else desperately needs this item, Martin decides to find the lock to which the mysterious key fits.