A professor and a group of students are killed when they consume mushrooms tainted with synthetic deathcap mushroom toxin. Sherlock questions the professor's former associate, Austin Harper, who said that although they had a falling-out, they had made their peace. The professor's stash leads them to another body, Charlotte Koenig.
Figuring that Charlotte was the intended target, they continue the investigation and discover that Charlotte was producing counterfeit erectile dysfunction pills in exchange for property. The property was strategically located to try to squeeze money out of a pharmaceutical company that Charlotte felt stole her idea and profited hugely off it.
Sherlock and company eventually discover that the very same Austin Harper was actually married to Charlotte and stood to gain millions by selling the property to the pharmacuetical company.
Meanwhile, Joan decides to deal with the noisy next-door neighbors and discovers that the neighbor has turned his residence into a short-term rental catering to the loud and rambunctious set, all as revenge for Sherlock and Joan's repeated disturbances of the peace over the years.
After a fire breaks out at the residence, Joan uses the brownstone's security cameras to prove that it was arson, thus ensuring the neighbor gets the insurance money. She also gives him a flyer on soundproofing, as well as a jar of honey from Sherlock's bees.