Sherlock Holmes Chapter One

This achievement is highly missable (but very easy). After starting the mandatory main story case 'The Muse From Abroad', you can return back to Stonewood Manor at any point. Waiting for you will be an agent of Mycroft. Speak with him, and refuse to take his case ('Love, Death and Cordona') for this simple achievement. You will be prompted to go back to Stonewood Manor after this main story case is over, and would be now you would “naturally” encounter this interaction, but you can do this at any point after starting it (credit: Tenreth)

You must turn him away for this achievement before accepting it! If you accept the case, you'll have to re-load an older save and make the right decision. You can still accept the case once you've turned him down for this achievement, so you don't need to reload.