Joan wonders if seeing old friends is a good idea while Kitty and Sherlock continue to drive her crazy. Why does she put up with it? Her friends are married or engaged and most are mothers. Is this the life she planned? Surgeon, sober coach, NYPD consultant? Who is Joan Watson anyway?

Whilst Kitty is reconnecting with old friends in England, Sherlock and Joan are sent undercover as a married couple to infiltrate a human trafficking ring. As the investigation continues, the duo must identify the sinister head of the organisation, who holds the secrets to the whereabouts of dozens of victims. As they do so, their fractured relationship is put to new tests.

Following the events of the last season, Sherlock and Joan work at repairing their partnership and, in so doing, discover a side to it which neither of them had anticipated. Whilst trying to come to terms with the true nature of their relationship, they find themselves embroiled in a case involving a ruthless killer, whose sights become fixed on Joan Watson. (Dedicated to RP&AW)

When Joan is injured by a violent suspect, she and Sherlock explore a new dimension to their relationship, which moves beyond platonic. The partners must deal with development of their relationship, the aftermath of their first night together, and a case involving the kidnapping of two young girls, which leads them to a new villain seeking to destroy them both.