Elementary Episode 1.12 M.

After examining a blood-filled crime scene devoid of clues, Sherlock is immediately able to identify their suspect as the London-based serial killer known as 'M.' Sherlock is intimately familiar with M's MO--in fact, he worked tirelessly to help the British authorities try and capture him when he lived in England. Joan is worried that M might trigger Sherlock into relapsing, but the detective assures her that M's return is a good thing--he now has a second chance to catch a criminal he was unable to stop last time due to his addiction. As Joan begins to realize that she will deeply miss working with Holmes once their time is up in a couple of days, Sherlock works in secret to find M without helping the police. When Joan realizes what's going on, she gets Sherlock to admit the truth--M killed his love, Irene Adler, and he intends to avenge her death.