A sniper kills and injures a number of people, and Morland Holmes approaches Sherlock to offer his assistance in solving the case. Though Sherlock is initially suspicious of the offer, he eventually gives in and takes advantage of his father's significant resources.
They initially suspect that the intended target was a lawyer named Ethan Paris, who was working on a major deal for Dynastic Energy. Sherlock and Morland eventually discover that this was incorrect, and the target was, in fact, a plumber from Staten Island.
They reasonably wonder why someone would hire an assassin to kill a plumber, eventually learning that the man was having an affair with the wife of a man who worked for Dynastic Energy. After interrogating him, however, they realize the guy didn't even know his wife was having an affair.
Morland connects the dots and learns that the wife had previously been cheating with her husband's boss, who was jealous and had the plumber killed. They are able to connect the boss to the assassin, and the man is arrested.
One of Morland's sources, an Interpol agent, attempts to blackmail Morland, threatening to tell that Morland is putting Sherlock in grave danger. Morland informs the agent of the... fate... of his predecessor, with whom Morland had a similar arrangement. The agent immediately backs down and departs.