There is still have no case against Stapleton, including a motive, but Holmes has a plan (which he keeps secret for now), and is still confident that Mrs. Lyons holds further useful information. Holmes tells Watson not to tell Sir Henry about the hound because he wants the baronet to keep the dinner with the Stapletons;
Watson however must get out of it to aid in the deception that they are returning to London and be on hand for the execution of the rest of the plan.