Watson comes home. From that, we can deduce that she's been somewhere else. That doesn't have anything to do with the episode; I just wanted to show that I can do some deducing myself. There's a bulb out in the brownstone's foyer, and a mysterious ski-masked figure demands to know where Holmes is. He's stolen her mace so she has nothing to spray him with. Watson runs, and then she slips and falls. It turns out to have been a trick by Holmes. He would like her to learn some violent skills, since she was in physical danger last week. Holmes is going to bring in a series of martial arts experts to train her. He'll also surprise-attack her. She's not entirely delighted by this news.
Bell is driving around at night and delivering a phone message to Gregson. You're not supposed to talk on the phone while driving, man. That's probably why he doesn't notice the car behind him until it starts honking. When it pulls up next to him, gunfire riddles Bell's car, and he runs it up a ramp so it does an A-Team flip. You know what that is, right? It's when a car hits a conveniently concealed ramp and flies into the air and lands on its roof. Everyone should know that.
In the police garage, Gregson decides that it was an MP5 that shot up Bell's car. Holmes says that they're pretty accurate weapons, so it was not a marksman. Because if it were, Bell would be dead. They were in three-round bursts. He thinks Bell should not have jerked the wheel because he would have been fine otherwise. He's also investigated Bell's history, and he has no shortage of possible enemies. This pleases Holmes -- e figures a policeman isn't doing his job right unless criminals want to kill him. Bell says he already knows who did it.
His answer is Curtis Bradshaw. He was an "aspiring drug kingpin" with a crew of almost a hundred. Bell was on a detail targeting him, and they put him away for eighteen months on a minor charge. Naturally, he recently got out. One of the detectives on the detail stole heroin from the evidence room and planted it on Bradshaw. But an anonymous source tipped them off. Bell put away most of Bradshaw's lieutenants, too. He has a picture of Bradshaw's car. And indeed, it looks like the one that was next to him. Also, Bradshaw vowed vengeance when he went to jail. People, don't vow vengeance in public. It just gets you in trouble. Gregson says Bell's obviously not on this case. But Holmes is! And also probably a police officer, but we never see evidence of that.
As they leave the police meeting room, there's a message for Bell from "Mr. Cheese," who he claims is a CI. Holmes observes that he's had sex with Detective Reyes, the attractive female police officer who delivered the message. Then he suggests that Watson box with Reyes, but she's skeptical of his motives. He claims that he just noticed that she boxes, and he wants Watson to be able to defend herself.