“A mystery I propose to explain." What is the mystery the speaker proposes to explain?

The speaker i.e. Gerrard proposed to explain the mystery of manipulation that he thought of in order to counter the intruder smartly.

The story was that Gerrard himself was a criminal just like the intruder. He very cleverly planned to raise the question of not meeting any trades people. In continuation to the story, he also planned to claim of committing a murder and escaping from it. However, due to the arrest of one of his men and the presence of evidence against him, he was forced to plan his escape that night and thus his bag was packed as he was ready to leave.