A blackmailing letter, without a doubt, Mr. Martin could see at a glance. But what on earth could the fellow have discovered? Fifteen years ago, when Mr. Martin moved from Praed Street to the Barbican, he had taken the utmost care to destroy every trace of the purpose to which that little cellar had been put. He racked his brains to try and think of anything that could possibly have been overlooked, but without success. Even those ingeniously contrived recesses in the walls had been torn out, leaving nothing but the bare brick. Mr. Martin had done the work with his own hands, not caring to trust anyone else with so delicate a matter. Yet, after all, in spite of all his care, he must have left some clue, which this infernal fellow Lacey had somehow blundered upon.