Chicago P.D. Episode 3.14 The Song of Gregory Williams Yates

Two notorious serial killers have recently escaped from prison. While one of the killers is in custody, Gregory Yates (guest star Dallas Roberts) remains at large. Benson (Mariska Hargitay) and Fin (Ice T) from SVU join Chicago P.D. in a race against time to capture Yates.

Unfortunately, the body count immediately rises in Chicago, and several murders all point to Yates. Yates delights in taunting Lindsay (Sophia Bush) with notes, texts and phone calls, goading her into catching him. Voight (Jason Beghe), much to Lindsay's dismay, attempts to keep her safe by keeping her on the sidelines.

When a box with a severed hand is sent to the district, addressed to Lindsay, it's discovered that Yates had tapped Lindsay's phone when she visited him in prison... and that he has effectively been tracking her every move. Yates' motives are also revealed - revenge against his biological parents, who abandoned him as a boy when he proved too much to handle.

After a furtive search that includes a trip to Lindsay's apartment (where Yates had initially been tracked), Lindsay gets a phone call from Yates that places him at his boyhood home. There, alone, Lindsay faces Yates and attempts to stop him from killing his next victim - his biological father. While she's unable to save the father, she escapes harm by shooting Yates when he lunges at her with an ice pick.

With the case closed and this chapter in her life over, Lindsay is consoled by Benson over a drink at Molly's, with Benson telling Lindsay that "my phone is always on."