7. One Way to Get Off

Complete story — includes the ending

Sherlock assists Gregson with a double homicide investigation that has the same modus operandi as a series of murders from 13 years earlier. That case led to the conviction of Wade Crewes, who is still in jail, and gave Gregson his big break. While Gregson looks for a copycat killer, Sherlock digs into the old case. He discovers that either Gregson or his then partner planted a mug with Crewes' fingerprints at a crime scene, which proved vital in obtaining Crewes' conviction. Sherlock questions whether Crewes is guilty after all. Crewes had no visitors during his incarceration, so an accomplice seems impossible, that is until Sherlock realizes a prison library worker named Sean Figuero taught the illiterate Crewes to read. Sean turns out to be Crewes' illegitimate son and his accomplice. After being frozen out by Sherlock, Joan pays a visit to his old rehab center in order to learn more about the enigmatic man now in her care.