I was fearful for a moment after Lestrade’s message to Joan that it may spur her into being more dubious of where she and Sherlock stand in the long run but she’s stronger than Lestrade is and was. Joan throughout their partnership has found a way to walk alongside Sherlock while Lestrade has always been content to linger in his shadow. Joan and Sherlock have a partnership, a relationship that is durable. It had a weak start but they rebuilt themselves on solid ground and into something that can’t be torn down by a few off-color words from a jilted ex-partner.
Joan watches as Sherlock fiddles with another explosive that he’s been sent by his friend to try and solve, exercising his mind. Joan asks what will happen this time if the clock reaches zero and he says explode—he asked for a real challenge this time. He acknowledges that she hasn’t left the room and asks her if she’s staying. She says she has faith.
It’s an off-the-cuff remark that she hardly draws attention to but Sherlock catches the poignancy and so do we audience members who have just seen the encounter that took place between her and Lestrade. It also is obviously a double entendre for their partnership—making it all the more touching of a scene and one that could have possibly been written with a touch more subtlety. Sherlock now listens, aptly, to Joan’s advice. Him reaching out to help Lestrade at the end is demonstrative of that and is something he may have never done had Joan not been there to help focus him. And Joan, despite the inconclusive nature of who she’s working with, can’t help but want to stick around because no matter Sherlock’s past and who he has worked with and left behind, she can’t help but trust in her abilities, her necessity to her partner, and how strong they are as a duo.