Watson and the fellows spring back into action when Mary's mother accidentally poisons herself while suffering from a rare form of dementia that is progressing with impossible speed. Meanwhile, Sherlock Holmes resurfaces after being presumed dead.
A year after the passing of his friend and partner Sherlock Holmes at the hands of Moriarty, Dr. John Watson resumes his medical career as the head of a clinic dedicated to treating rare disorders. However, he soon finds that his old life is not done with him yet.