Alfredo and Sherlock reunite this week, but this reunion may change the dynamic of their relationship for good. Watch a sneak peek of Elementary season 3, episode 21.

Over three seasons, Elementary has taken Sherlock Holmes’ trademark insular personality and sent him on course to not only solve crimes against the backdrop of New York City, but also develop meaningful relationships, the first of which is quite obvious. Joan Watson grew from her position as sober companion to his partner and, though he would likely cringe at the term, best friend. But another person became intertwined with his journey through sobriety, his sponsor Alfredo Llamosa.

Related: Elementary season 3: Getting to know Sherlock’s sponsor Alfredo

While he has mainly been a behind-the-scenes player throughout the series, Alfredo is one of two people Sherlock confides in and trusts. It has taken until season 3 for Sherlock to acknowledge what a working definition of friendship means to him — two people finding the best aspects between them and moving toward drawing those traits out of the other. Sherlock views his ability to spot a problem a mile away and Alfredo’s way of making someone talk openly and feel comfortable are two of their best qualities.

But what Sherlock does not anticipate in the following scenario is that he and Alfredo are not on the same level of understanding where the line is drawn between their relationship as sponsor and sponsee ends and their possible friendship.

Sherlock confronts Alfredo

Though Sherlock’s concern for Alfredo is buried deep under his exposition of Alfredo’s dealings with his former employers, there is only so far someone can be pushed. Taken aback from this retort, we’ll have to see how the rest of their arc plays out in this week’s episode.

Besides Alfredo’s clandestine carjackings, the case that absorbs Joan and Sherlock’s attention involves a kidnapping in an ambulance that leaves two paramedics dead in its wake.

Sherlock goes fishing

DIY shootings

Watch Elementary season 3, episode 21, “Under My Skin,” Thursday, April 23 at 10:00 p.m. ET on CBS.