While helping Joan learn deductive skills, Sherlock is approached by Samuel Hagan, a wealthy man suffering from an incurable disease that is destroying his mind. Even though the disease is genetic, Hagan is certain that someone gave it to him as part of a corporate power play. Sherlock is unconvinced and refuses the case. After Hagan kills his driver, however, Holmes finds himself intrigued enough to take the case. Holmes begins to believe Hagan when a scientist at a corporate genetics laboratory tells him that giving someone the disease is theoretically possible, but the trail runs cold when she turns up dead. All the evidence, including DNA results, point to the killer being an ex-con unrelated to Sherlock's case. But can it really be that simple? Meanwhile, Sherlock sends Joan on an errand to a suspicious dry cleaning establishment in order to test her ability to solve a case without his help.
