A man lies dead in the street. Sherlock speaks at his sobriety meeting. Afterwards a man asks him if BrainAttic, a blog, is written by him. Sherlock says no and he suggests Sherlock check it out.
Captain Gregson calls Sherlock. A police officer has been murdered. Alec Flynn was ambushed by a masked stranger and shot. Sherlock examines his gun - it's a toy gun.
The video from Flynn's dashboard camera shows the suspect never touched Flynn's weapon. Bell and Joan talk to Flynn's wife. She admits that he had become addicted to OXY, but was supposedly clean again.
Sherlock is upset when he discovers someone is sharing what he talks about at his sobriety meetings in their blog. While the quotes aren't attributed to him, they are things he has shared in the meetings. Kitty says she will find out who the blogger is.
Joan gathers footage from gun shops and discovers that Officer Flynn bought the toy gun himself.
Sherlock and Joan meet with Gregson. They believe Flynn sold his service weapon to fund his addiction. Sherlock believes Flynn was taking guns from the armory, selling them and replacing them with fake weapons. Gregson confirms 30 guns are fakes.
The ATF helps them identify Flynn's killer as Niko Buros. He sells guns to cartels in Mexico and he sells their drugs here.
A cop sits in his car while his partner goes into a convenience store. A man comes up and shoots him. It's Buros.
They look for a connection between Officer Flynn and Officer Hateem. Both had ties to the armory, although Hateem comes up clean.
Sherlock confronts BrainAttic. He says BrainAttic is helping people and refuses to take down his blog. Sherlock threatens to tell his wife that he is having an affair if he doesn't.
They can't figure out who Six is. Joan notes that Flynn played football and sometimes players refer to each other by number.
Six is being interrogated by Gregson and Sherlock. He admits that Flynn sold him his gun, which he sold to Niko. Niko got upset when Flynn stopped selling guns but figured out how to use Flynn in a different way.
Sherlock realizes Niko is going to rob the armory, but needed to draw cops away, which is why he shot Officer Hateem.
Buros robs the armory, stealing a million dollars worth of military grade weapons.
Sherlock blames himself for not realizing this earlier. Kitty tells him the BrainAttic website only says "I'm sorry" now.
Joan figures out Nikos is cutting up furniture to hide the guns. They realize he needs to move the weapons now, by ship. They raid Niko's hiding spot. Hundreds of police wait outside as Bell arrests Buros.
When it is Sherlock's turn to speak at his sobriety meeting, he declines to share.