Outaishi Denka wa Koukyuu ni Uranaishi wo Goshomou desu

One day, the duke's daughter Forte, a fortune teller, is falsely accused and threatened by the crown prince's aides to "find the mastermind behind the murder of the prince and the princess among the Three Princesses of the Inner Palace." If she fails, she faces imprisonment, so she disguises herself as her maid and begins investigating the Inner Palace with the help of the mysterious Black Knight Alto. Who exactly is he..!?.