Molly Hooper must help Sherlock and John capture a diabolical serial killer before it is too late. Who will the next victim be? Meanwhile Sherlock and Molly's rather rocky relationship continues to be a roller coaster ride. Will these two ever get their act together?

Alena has moved from the U.S. to London to escape her tragic past in hopes that she can begin a new life. When she moves into a flat that makes her the new neighbor of Sherlock and John, her life is not only completely changed, but unexpectedly she finds that her heart might be able to learn to love again. Sherlock/OC.