In the comic books, there's something called the "Royal Flush Gang," which often doesn't have the same members from appearance to appearance. It's usually five low-level dummies in card-themed outfits, but they sometimes have powerful sponsors. They sort of show up in this episode as bank robbers, but the card themes are restricted to hockey masks with some card symbols on them. Also, there are only four of them, which is not a proper flush no matter how you look at it.
Oliver doesn't want to sully his arrows by fighting bank robbers, but Dig is enraged at the injustice. Also, a policeman got shot during one of the robberies. So Oliver gets shamed into investigating them. And he turns out to be much better at that than the police, because after he breaks into the police station to steal the evidence (on a thumbdrive), it takes him about five minutes to locate the culprits. It's a family of bank robbers being led by a man who used to work in the very steel mill where Oliver now has his Arrowlair! Well, they never actually say it's the same place, but it probably is. Look, the point is that he worked for the Queens but got laid off by Oliver's evil father. So there's enough of a connection to make Oliver all obsessive about the case.
The family continues to rob banks in Starling City, even though you'd think it would be a good idea to mix it up a little and try a city that doesn't have a famous vigilante in it. To keep up with them, Oliver has to repeatedly walk away from his mother while she's telling him how much she misses him. But he eventually stops the bank robberies, although the man that was fired by Robert Queen gets killed. And Oliver has a crummy diner hamburger with his mother, which makes everything okay.
In non-Oliver news, Laurel's legal aid group ("CNRI") is in financial trouble, and Tommy realizes he can put on a fundraiser to make her like him. This is weird because Tommy and Laurel already decided to date a couple episodes ago. Also because they keep reminding us that Tommy's a billionaire, so you'd think he could just bankroll this tiny legal aid operation himself. Anyway, the real awkwardness comes when Laurel goes off to dance with a handsome neurosurgeon, leaving Tommy alone with Thea Queen, who's suddenly in love with Tommy and jealous of Laurel.
I'm not recapping the introductory segment anymore. Go read the other recaps if you're so interested in the premise of the show. Tonight's actual story begins when some people in hockey masks walk into a jewelry store and punch people in the face and shoot automatic weapons around. One of their masks has an Ace of Spades on it.
So naturally the first assumption is that these are the Royal Flush Gang. In comic books, the Royal Flush Gang can be a lot of different groups, but it's basically a five-person group with outfits themed to playing cards. If you'd like to see them in action, I recommend Googling the comic The Craftsman Bolt-On System Saves The Justice League.
The Jack and the King go into the safe while the Ace stays out with the hostages. But one of the hostages is a cop! He starts to sneak his hand down to his ankle holster, but a lady hostage whispers for him not to draw attention. He won't listen, so she starts talking louder. This results in the Ace shooting the police officer. And that results in the police arriving ahead of schedule, while the Jack is still in the safe. The King seems unhappy with the Ace. The Ace stares at the woman. Well, as much as he can with his entire face hidden with a hockey mask, anyway.
After the credits, the hostages run out of Starling Trust Bank (which is where this was happening!), all wearing hockey masks. The cops rush into the bank to find a hole in the safe floor. See, I assumed that the gimmick was going to be that the robbers were among the hostages, which would explain the masks. As it is, the only explanation is that the bank robbers always bring a few dozen hockey masks along on a job. I bet that would be a good way to track them, right? You just ask around to find out who ordered a giant load of playing-card-themed hockey masks.
Anyway, the bad guys are at a manhole arguing about whether it's a good idea to shoot police officers in the middle of your operation. So their getaway plan was to use a portable jackhammer to drill a hole in the floor of the safe, then get into the sewers. Considering that one of them said they only had three minutes, I'm not sure I buy it.
Hey, escrima! Oliver and Mr. Diggle are doing this thing where they each hold two sticks and they're knocking them against each other. Oliver tutors Diggle on the concept of "variable acceleration," because of course he's the expert on everything. He says learned escrima from someone named "Yao Fe," but he won't go into detail. He will, however, whack Dig around because it's important to him to show how much better than Dig he is.