Arrow and Arsenal approach a house from different sides and call in that they are in position. As does Diggle from the van. They’ve tracked the boomerang killer to this address. Arrow notices that the house is rigged to explode, so both archers back up and then trigger the bombs with their arrows. Once it is safe, they enter and begin to search the house. Suddenly, they are face to face with a SWAT team, and both side freeze into a standoff. Ollie realizes that the team is not police, and deduces that they are in fact ARGUS. After an argument with one of the ARGUS men, the SWAT team leaves, with a warning to Arrow to let it go, ARGUS will handle this killer. As expected, Arrow has no intention of stopping.
In the Foundry, Felicity looks up information on the killer’s latest victim. Lyla has refused to tell Dig anything about ARGUS’ investigation, which Ollie is rude about.
Waller shows a bound man to Ollie. She needs information from this man to prevent a bombing and expects Ollie to torture the man to extract it.
Caitlin show up in Felicity’s office at Palmer Tech. At first Caitlin plays it off as her offer to help find Sara’s killer, but Cisco reveals that they want to see the Arrow Cave. Felicity warns them not to call it that.
At the Foundry, Ollie watches Cisco like a hawk and prevents him from touching anything. Cisco is already trying to figure out how he can improve things.
Diggle follows Lyla into ARGUS as she continues to deflect his questions about the boomerang killer. Not long after, the boomerang killer uses one of his victim’s IDs to enter ARGUS and the slaughter begins. ARGUS goes into lockdown. Diggle calls Ollie for help. Lyla gives Dig a gun and they join the fight.
Thea is walking outside and talking to Chase on her cell phone when a red blur shoots past her. The Flash has arrived in Starling City.
Arrow and Arsenal get to ARGUS and join the fight. The Flash arrives just in time to save the day, but the boomerang killer g escapes. Arrow demands answers from Lyla, who finally gives them. His real name is Harkness, and he was a member of the Suicide Squad. When a mission went badly and Weller was out, Lyla made the decision to terminate the members of the squad. Harkness somehow survived. Arrow offers Lyla protection.
At the Foundry, Barry is using the salmon ladder at top speed. Cisco tries next, but can’t even manage a pull up. Barry decides to head out for sushi. While he is out, Lyla and Diggle show up with a shattered boomerang. Barry returns with the sushi and his mask off, assuming Diggle told Lyla about him. Of course, Diggle didn’t, but now Lyla knows the identity of the Flash. Ollie pulls Barry aside and tries to convince him to go home. But Barry wants to help, and promises he has taken the advice Ollie recently gave him in Central City. Ollie finally agrees that Barry can help, but they are doing things Ollie’s way. Barry immediately proves his worth by reassembling the boomerang in seconds. Cisco and Caitlin work on the boomerang and recognize the signature of Klaus Marcos, who Lance arrested before.
At the police station, Laurel and her father are once more arguing about their shared cases. Barry and Ollie show up and greet them. Laurel asks Ollie about his trip to Central City. Lance shares that Marcos was arrested for identity theft and has ties to the Russian Mob.
The team goes to find Marcos, but while Arrow checks on Diggle, the Flash ties up all the mobsters, then grabs Marcos. Arrow shoots Marcos, trying to torture information about Harkness out of him. The Flash is horrified, but Marcos hands over an encrypted cell phone. The Flash delivers Marcos right to Lance’s office.
Barry is furious about Ollie’s methods, but Ollie defends them.
Back at the foundry, Lyla asks Ollie about Barry. She understands what it is like to have to make hard decisions for the greater good and not be able to make others understand. Felicity easily hacks the ARGUS spy satellite to find Harkness, using info from the cell they collected. Cisco says he can track the signal from his phone, so he joins Arrow, the Flash, Arsenal, and Diggle in hunting Harkness down.
Ollie takes his bow and arrow in his hand, trying to psyche himself up to torture the prisoner, who taunts him. The prisoner doesn’t think Ollie will torture him. And he’s right, a bomb explodes in a nearby building before Ollie can do anything.
In the van, Cisco tries to chat with Diggle and Arsenal. It is mostly a one-sided conversation, during which Cisco admits that Barry can run on water. Cisco says that he thinks super powers are given out to help people deal with the craziness of the world.
Arrow enters a bar, followed by the Flash. Things go south right away, and Arrow saves the Flash. Arsenal and Diggle head inside, leaving Cisco in the van. They learn that Marcos was paid to give Arrow the phone, it was all a trick.
Harkness uses the phone to find the foundry where he attacks the girls. Lyla quickly starts grabbing weapons to defend Caitlin and Felicity. After a short fight, Harkness manages to get Lyla in the chest with a boomerang. Felicity throws an explosive at Harkness, who escapes. Felicity and Caitlin hurry to see if they can save Lyla. The guys return, and Barry hurries Lyla to the hospital.
Weller meets up with Barry as they examine the wreckage. He is distressed, and she does not help matters at first. Eventually, she talks to him about extremes, words he echoes later. She tells him that torture is an art.
Roy, Felicity, and Cisco share a table at Verdant, where Thea serves them. Cisco likes Thea and asks the other two who she is. But upon hearing she is both Ollie’s sister and Roy’s ex, Cisco gives up without a fight. Caitlin arrives to update them on Lyla’s condition. Cisco and Caitlin are stunned by what they have seen in Starling City. It was almost a game to them, playing sidekicks to a superhero. Now they get how serious it really is.
Ollie is in the foundry, blaming himself for what happened to Lyla. Barry tells Ollie that it is not his fault. Ollie confesses that he is willing to do what’s ugly, but he loses a piece of Oliver Queen every time. He thinks at this point that he is only the Arrow. Barry sees it differently, and tells Ollie that his humanity is what saved him. Felicity interrupts, they’ve found Harkness.
In a subway station, Arrow and the Flash go to confront Harkness, who tells them that he planted five bombs around the city. They have 90 seconds. The Flash hurries off to deal with the bombs while Arrow stays to fight.
The Flash finds a bomb and talks to Felicity, Caitlin, Cisco, and Roy about what he sees. They realize that they need to defuse all five bombs at the exact same second, or they all go off.
Arrow manages to use arrows to pin Harkness to a pillar.
One by one, the Flash grabs his friends from the foundry and drops them off with a bomb. Once all five of them are at their respective bombs, they count to three and cut wires. It works.
The Flash returns to the subway station in time to hear Harkness call Arrow weak. Arrow disagrees, and references his humanity.
Diggle sits by Lyla’s hospital bed. When Lyla wakes up, Diggle proposes marriage. She accepts.
At the foundry, Caitlin is pretty sure she can get results from the arrows that killed Sara. Ollie tells everyone that Harkness is now sharing a cell with Slade Wilson. He then reveals that he had Felicity make a station for Barry’s Flash things in the Foundry, so they can team up again in the future. Team Flash also has a gift for Ollie, Cisco presents him with a new Kevlar jacket, green hood attached.
Weller offers Ollie a second chance with a new prisoner, someone who works for China White. This time, Ollie does not hesitate to begin torturing his victim.
Barry and Ollie walk together. Ollie wants to find out who would win in a fight between them, once and for all. They argue until they arrive at the space Ollie has chosen for the bout. And just as they are about to begin their own version of superhero thunder dome, blackout.