Hannah asks Joan to help solve a string of robberies. A man comes across a stalled car on the railroad track. He manages to push the car safely off the tracks. He thinks the driver is drunk but he's dead. The deceased, Garrison Boyd, is a professional doubter who Sherlock admired. Thanks to the Church of Atomicism and their surveiling of Boyd, he is able to find photos of him aruging with Collin Eisley. Bell and Sherlock interview Eisley who claims that Boyd accused him of making a woman believe her home was haunted. Joan and Sherlock visit Mrs. Renfinzger who believes her husband, Harry, is haunting her. She recorded him speaking and plays the tape for Sherlock. He realizes the man is speaking Arabic and can faintly hear digging equipment. He finds a tunnel in her neighbor's basement that exposed Ruby, the transatlantic data cable connecting the United States and Europe. Meanwhile Joan deduces that in all three robberies the same candy vendor had visited them prior to being robbed. She has photos of the alleged robbers at the candy warehouse. She tells Hannah that the police will likely surveil the warehouse so they can see where the drugs go. Sherlock has a lead on someone who can identify the man who dug the tunnel. To his surprise, she claims the man is her neighbor. Unfortunately, he overheard their conversation and started his apartment on fire. What terrorist plot did Boyd uncover? Watch Elementary Season 3 Episode 20 online to find out the answers.