Last week’s surprise ending sets up a much bigger story for Joan Watson in the second arc of Elementary season 3.

Joan Watson has come into her own as an investigator. She took a backseat to most of the detective work in “Hemlock,” giving Sherlock a much needed case to distract him from Kitty’s absence. Joan’s growing presence in the brownstone is for Sherlock’s comfort, but it is not enough to allow either of them to grow. Especially not as long as Joan remains in a relationship with Andrew.

The fling with Andrew was more of a thorn in her side than an entanglement that gave her happiness. As she prepared herself to pluck the irritant out, she learned more about herself in the process. Andrew’s father was impressed with her commitment to changing her path until she found a passion that made her happy. And Sherlock even helped by bringing to her attention that it is the unconventional nature of her journey that makes Joan happy. Andrew does not fit into that journey.

That is until now.

Elementary season 1, episode 12, “M,” set the second arc for the inaugural season in motion. The episode closes with Sherlock sitting across from a single notecard with the name “Moriarty” written on it. While the name did not come back into focus until the final three episodes of the season, the fuse was lit.

In Elementary season 3, episode 13, Joan’s fuse is lit when her own arch nemesis, Elana March (guest star Gina Gershon), resurrects her presence and sends someone to poison Joan. However, the toxin intended for Joan is slipped to Andrew by mistake.

Related: Elementary season 3, episode 13 recap: “Hemlock”

To keep a procedural show from getting stale the B story needs to find a way to introduce new obstacles that compliment the overarching case work. Elementary‘s third season threw Kitty Winter into the mix of Joan and Sherlock and ripped her away just as the things became too comfortable. This set Sherlock back into the same state of limbo he experienced once Joan decided to move out of the brownstone at the close of season 2.

Andrew’s love was unrequited, but Joan cannot remove herself from another innocent life lost because of her hand. Instead of running from her career, Joan has Sherlock to step up to the plate and find a way to support his friend as she has done for him many times before.

The story could easily revert back to season 1 levels of conformity with Joan and Sherlock heading back into their agreement to be partners with the NYPD. What the story needs is a shock. Something that gives Sherlock a new purpose and offers Joan a way to surge beyond her investigative plateau. Elementary smartly builds in a parallel, yet different path for Joan to follow. Sherlock has been running fifty paces ahead of Joan for two and half seasons. It is time for Joan to quicken her pace and take the torch for a bit.

And this week Lucy Liu takes the helm in more ways than one. Not only will Joan have a major story arc, but Liu takes the director’s seat for “The Female of the Species.” This is her second episode as director. Her first episode was season 2, episode 22, “Paint It Black.”

Watch Elementary season 3, episode 14, “The Female of the Species,” Thursday, February 12 at 10:00 p.m. ET on CBS.