When a research scientist is killed by a helium overdose, Joan and Sherlock investigate the world of pharmaceuticals. An anonymous writer named Adam Peer tries to ruin the credibility of a breakthrough device called "The Hound" that will detect cancer but a blow of air.

In a plot twist, The Hound’s main inventor was the one behind both murders and the false Adam Peer post.

Bell finally gets to return to field duty and has some awkward tension with Joan. All the while, finally having some "bro" time with Sherlock. Bell also helps one of his childhood hero’s believe that he’s made a difference. Only “the legend” ends up killing a bad man and in turn gets himself killed breaking Bell’s heart.