Two weeks later John arrives home with a slight bruise across his cheek and Sherlock knows that something is wrong. Not just wrong, very wrong, and he is fairly sure that the root of the problem is John's new girlfriend and the strange proximity of John's dates with her and the mysterious appearance of his new and frankly unpleasant injuries. The fact that John claims that his bruised cheek is due to a collision with a door at work only encourages Sherlock's deductions. Walked into a door, John's excuses aren't even inventive.
