Ingrid's lies come back to haunt her when her sister Gigi needs to be treated by the team after she develops side effects to the secret treatments Ingrid put her through. Also, Watson and Mary bond over a tragedy.

When part of the team falls ill after becoming targets of a sinister plot, it becomes an all-hands-on-deck situation to save them before it's too late. Meanwhile, Watson announces a new ambitious project that he has been working on.

Watson and the team are faced with an impossible choice when two lives hang in the balance, but they only have one vial of antidote available. Meanwhile, Shinwell races to help the team, and Watson goes head to head with Moriarty.