Synopsis for 3×20: After an infamous debunker of the paranormal turned up dead in his vehicle, Sherlock set out to find his murderer and instead unearthed a financial scheme. Joan decided to help out Gregson’s daughter, Hannah, investigate a case that had been put on the back burner.
As a gentleman drove home, he was passed by a rather unruly driver. However, as he came up to a set of train tracks he realized just in time that the other car was stuck on the tracks. In a moment of heroic action, he got out of his vehicle and helped push the car out of the way just in time to avoid getting smashed by a train. However, it turned out his efforts were really for nothing except preserving a body because the driver was very dead.
The dead guy was a “debunker” of the highest order. He was a doubter of a lot of things and wrote a blog that Sherlock followed. While talking with the medical examiner, Sherlock and the examiner both came to the conclusion that the man’s head had been bashed in with a crowbar. Strangely enough, there was some sort of ceramic tip that had been shoved beneath his eye. It turned out it was the tip of a gnome hat that he must have landed on, on his way down.
Joan parted ways for part of the episode as she joined with Gregson’s daughter, Hannah, to help her with a string of robberies in her area.
Sherlock’s investigation led him and Bell to a modern cult that had been upset with the debunker’s statements about them. While Bell stayed to interview the leader of the organization, Sherlock walked off to talk to a young woman. As he returned, he explained his brief absence and admitted that he had convinced the woman, Aria, not only to promptly leave the church but to tell him all about the processes. While it made the cult leader a bit irate, it unfortunately also cleared the cult of any suspicion.
Sherlock and Bell made their way to the next stop, which was at the house of a real estate investor who had been the last one to see the dead debunker. Apparently the debunker had harassed him and accused him of gaslighting an elderly woman who had turned down an offer on her house. Of course the real estate investor denied everything and even provided an alibi for his whereabouts the night it happened.
With that lead mostly gone, they turned to the elderly woman in question. She explained that the noises she’d been hearing were real. It was her dead husband’s ghosts, angry at her for all of the things she’d done with other men without telling him. Now that he was dead, he knew it all, and had been haunting her as a result. Of course Sherlock didn’t buy it, but upon further investigation did come to the conclusion that she was definitely hearing voices. It just wasn’t from spirits.
The voices seemed to have come from the neighbor’s basement, and upon further investigation Sherlock found that someone had been digging a hole into the foundation of the old woman’s house. He, naturally, crawled into the hole and left Watson behind to find what they could have possibly been digging for. What it turned out to be was a transatlantic data cable, buried deep beneath the ground.
Joan finished up working with Hannah and gave her the case to turn into the detective who had been working on it. She and Sherlock discussed it and Sherlock mentioned that he thought Hannah was a good cop, but not a good investigator. He questioned whether or not Joan should have helped her, and even though Joan was insistent it was nothing, even she began to doubt.
Following leads, Sherlock tracked down the neighbor of the man who had been digging the hole to get to the data cable. While talking with the neighbor he quickly realized that the man he sought was right across the hall and was in the process of dousing his apartment in lighter fluid. Moving quickly, Sherlock pulled the fire alarm and busted into the apartment. He’d hoped to have a talk with the man, but he lit the apartment on fire and took off out the fire escape. While Sherlock was able to recover some things from the apartment, it wasn’t a lot.
They did find a strange device that appeared to be some sort of data grab that was made to be attached to the cable.
Their suspect led them right back to the real estate developer from earlier in the episode. He apparently had done time for insider trading at some point and had changed his name to leave his past behind. They thought he’d paid the guy to dig the tunnel to gain some sort of investment advantage, perhaps get insider information once again. However, the real estate fellow, Colin, claimed that he would have no interest in that information as part of his penalty was to have his investments put into a blind trust. He had no say in where his money could be invested.
Joan confronted Hannah about the case she’d helped her on, and Hannah admitted she’d essentially been using Joan to get more credit thrown her way. She had her eyes on the Sergeants Exam and an accommodation for solving a string of robberies would give her extra points toward it. Sherlock’s musing on the young woman were shown to be correct and Joan was clearly upset.
Returning to the box, Sherlock and the team found that it was doing absolutely nothing. It was not collecting data, or transmitting it anywhere. All it was doing was passing the data through more cables and circuits before putting it back into the bigger data cable. No one could figure out why someone would want to create a device of this sort, until Mason and Sherlock both worked on it and figured it out.
The box takes an extra 4 miliseconds to push data through, which was the point. It was taking longer for the data to get from point A to point B. It just so happened that the place that managed Colin’s blind trust didn’t have a prime spot to get information as quickly as the other places did, which Collin didn’t have access to. So he tried to slow down what the competitor’s had access to, in order to give his own investments a little bit of a boost.
Of course the final nail in his coffin came in the form of a painting. He’d had a very expensive Picasso on his wall earlier in the investigation that was gone. It had been used as payment, and the moment it was flagged as stolen so it would have no value, the engineer he’d been working with gave him up. He was sunk, and so was his get rich scheme.
Gregson and Joan had a talk about Hannah, and he suggested Joan not help her anymore. She is what she is, and she wants what she wants, so in Gregson’s opinion it was best to leave her. He loved her and wanted the best for her, but she had to forge her own path.