Sherlock returns after three months away. Watson has a house guest, Kelsey. Joan is planning to adopt Kelsey's baby. A body is found encased in cement. There is no sign of Michael killing while Sherlock was gone. Joan admits to Sherlock that she wants to adopt. An RF chip in the dead woman's bracelet helped her identify her as Lauren Wexler. Levi Salinger, her ex-boss, is a suspect, but he was at a yoga retreat when Lauren was killed.  He suggests they check out a man named Troy who was arguing with Lauren. Sherlock figures out where Troy Roselli lives. Troy tries to run from Marcus while Sherlock and Joan break into his garage. They find a completed aquatic craft like she was supposed to be building for Salinger. Troy was helping Lauren build it. A security guard from the construction site threatened Lauren. Sherlock thinks the construction site may be dumping hazardous materials into the Hudson River. Sherlock figures out that the construction company, Topaz Valley Industries, was siphoning off sand instead, enough so that the nearby bridge was on the verge of collapse. Sherlock suspects an Indian group known as the Sand Mafia. Sherlock, Lauren, and Marcus interrogate Vikrant Jindal. He claims he received reports that the bridge was in no danger. Sherlock tries to allay Kelsey's concerns about Joan. The engineer who wrote Jindal's reports is found dead in Jindal's signature manner, but Sherlock thinks Jindal is being framed. He points out the victim was bound with a leather belt, anathema for a Hindi. Lance Pickering, who issued the permit for the sand dredging, committed the murders of Lauren and the expert. Kelsey tells Joan she's keeping her baby. Joan and Sherlock inspired her to try to have it all.