In Mr. Mercedes, a demented serial killer taunts a retired police detective with a series of lurid letters and emails, forcing the ex-cop to undertake a private, and potentially felonious, crusade to bring the killer to justice before he can strike again. Based on the bestselling novel by Stephen King.
Bill almost shoots a neighbor boy, and Pete and Ida both advise him to get his health checked. Meanwhile, Brady gets revenge on his tyrannical boss, and Janey Patterson agrees to have Bill investigate the suicide of her sister Olivia.
Bill tries to throw Mr. Mercedes off by claiming that the police have captured the "real" Mercedes killer. Meanwhile, Lawrence discovers what his son Jerome is doing for Bill, and Pete expresses his concerns to his ex-partner.
Janey's mother ends up in the hospital, and Bill meets Janey's niece Holly. Meanwhile, Brady begins to feel the pressure during his interview with the corporate men, after Deb decides to clean up her life and goes for a drive.