A phone rings in an empty apartment.
At the Federal courthouse, a bomb-defusing robot rolls into the building. It approaches a duffel bag in the empty building and confirms that chemicals indicating a bomb is present are present. The tech operating the robot has it open the bomb and try to deactivate the bomb. However, when it opens the bomb, there's nothing inside. The techs go inside and the robot explodes.
As the team fly back from a mission, they play Truth or Dare and Wilt has to wear his underpants on the outside. Desi agrees to Truth, and the others ask her what she owes Jack. Before Desi can answer, Mac gets a call from Charlie. When Mac takes the call, Charlie tells him that they have a situation and explains that the LA courthouse got bombed and two techs were killed. The person responsible created a false alarm and put the bomb inside of the robot. Charlie asks Mac to help them check out the scene, and the team agree.
Charlie goes into the building looking for a secondary device, and his radio cuts out. A man in a cop uniform approaches Charlie and says that he can't leave. He addresses Charlie by name, draws a gun, and shoots Charlie in the leg. Charlie goes down, and the man injects Charlie with sedative.
When the team arrive, the LAPD commander explains that they lost contact with Charlie five minutes ago. Mac insists on going in. Matty and James monitor the situation from the war room. Mac puts on a protective suit and goes in, and finds blood drops on the floor that happened after the detonation. The drops lead to the fire escape, and Riley says that she got a lock on Charlie's phone and confirmed that he's at the Aliso high-rise two blocks away.
When Mac and Riley drive there, they track Charlie to the 20th floor. Riley finds Charlie tied up in an outside elevator that technicians are trying to pry it. Mac stops them and presents his ID, and the operations manager confirms that Mac is a Federal agent. The others step back and Mac pries opens the doors with Riley's help. Charlie is bleeding out, and shakes his head to stop Mac from coming in. Mac sees sensors on the elevator walls and goes to the floor above. He sees a bomb on the elevator roof and goes down the cable. Mac confirms that someone attached a homemade cable-cutting device to the elevator cable, and they can't rescue Charlie without triggering it.
Mac goes back inside and uses his knife through the door to cut Charlie free. Charlies binds his wound and explains what happened. Mac tells him about the cable-cutting device and admits that he doesn't have a plan yet. Back on the roof, James tells Mac that the construction of the device is similar to the bomb on his SUV. His radio cuts out and Riley confirms that the device is sending a transmission interfering with their signal, going to a nearby hotel. The others take an assault team to the nearby building where the signal is transmitting and finds dozens of barrels of hydrochloric acid in the basement. If they open burst open, they will bring down the building. They're all wired to a control box similar to the one on the envelope. Mac gets an unknown caller, and the man who shot Charlie says that he's a great admirer of Mac. He says that he wants Mac to choose between Charlie and the innocent strangers in the hotel. If Mac does nothing then both devices will detonates, and gives Mac to the top of the hour to make his choice.
Mac tells Matty and the others, with twenty-three minutes until the top of the hour. The barrels have the same sensors as the elevator, and the devices are cross-wired so that disabling one box will generate the others. The signals are randomized to prevent Riley from intercepting the signals. Riley assures Mac that he's got it, and James heads there to meet with the team.
Mac figures that he has no options, and James tells him not to breathe. Charlie watches from the elevator, and James tells him that sometimes they only get to choose between bad and worse. He admits that it's time to live with the consequences of bad choices, and tells Mac not to let the bomber's motives cloud his instincts. Mac focuses and works out Atwood's Machine Problem to work out to catch the falling elevator. He gets a second elevator cable and splices it into Charlie's elevator. Before James go, Mac thanks him for the talk earlier.
Once James leaves, Mac explains to Charlie they're going to have the second rope catch him when his elevator falls. James and Desi go to the hotel and James prepares to disable the control box once Charlie is secure. As Mac climbs down to Charlie's elevator, the man calls and asks if he checked the anchor points on the second rope. There's a secondary cutting device, and Mac tells Riley, Wilt, and Charlie that his plan won't work. Charlie realizes what's going on and says that if the roles were reversed, Mac would sacrifice himself. He has Mac promise to get his killer, then punches the wall to set off the sensors. The cable-cutting device releases the elevator and it crashes below.
As the EMTs take away Charlie's body, James tries to reassure Mac. Wilt wonders what happens next, and Mac says that he's going to find the killers. He checks the elevator wreckage and finds a component from the cable cutter. The serial number has been filed off, and Mac works out how to bring it out using an electrolyzed acidic solution. Back at the war room, Matty confirms that the order was purchased through a shell company owned by Elliott Mason, a former FBI agent. Mason started out undercover and moved to counterterrorism, and then started the ATU: Asymmetrical Tactics Unit. The ATU performs operations designed to disrupt and disorient enemies. Mason quit a year ago and disappeared, and James says that they have no idea why Mason has a grudge. Riley has checked a shipment of the components to a shop in East LA.
The team and an assault team arrive at the squad. Matty reports that there's movement inside, and Mac breaks in. Mac and Matty head to the one room where the movement is coming from, and the assault team throws in a tear gas grenade. Desi kicks in the door and tells Mason to freeze, and Mason congratulates Mac before Mac punches him. The assault team takes Mason away.
At Phoenix, Mac tells Mason that he needs to know if he's planted other devices around LA. Mason says that he hasn't and explains that his son was assigned to overseas duty. He was tasked to obtain an intelligence asset, and his unit was ordered to engage an enemy group. It was all a distraction to let the asset escape, and Mason's son was killed. Mason says that Charlie and his son were sacrifices, and asks if Mac feels proud of Charlie's sacrifice. He says that he feels the same way about his son and it's the right feeling. Mason then explains that the man who ran the extraction op was James.
Matty and James watch, and James confirms that the story is accurate.
Mason tells Mac that he wanted to make James see Mac fail and suffer to get payback. He promises that he will destroy everything that James loves to make sure he gets what James truly deserves. Riley tells Matty and James that Mason doxxed them, and James figures that they caught Mason too early.
Mac tells Mason that he dishonored his son and Charlie didn't deserve to die. He says that Mason will spend the rest of his life behind bars. Mason removes a fake tooth with incapacitating gas, detonates it, uses the water bottle for oxygen, and escapes through the vent before security can get there.
Phoenix goes into lockdown, and Desi takes James to a safe room. The medics revive Mac and Wilt says that Mason took an access key. They go to the war room, and Riley confirms that Mason has broken into the server room. The coolant system goes down and the servers overheat, setting off fire alarms. Mac has Riley take them out of lockdown but she can't. The fire-suppressant systems are off-line, and the team realizes that everything is there that Mason wants to destroy. Mac and Riley go to the server room while Desi and Wilt look for Mason.
The servers are on fire and the toxic smoke spreads throughout the building. Mac spots Mason and tells Riley to go upstairs, and he goes after Mason.
The agents manage to get a shutter open and everyone evacuates. Desi and Riley go to get James.
In the safe room, Mac improvise a gas mask. Outside, the agents report to Riley and Desi that they've been locked out, and Desi goes to get something to break down the door. Riley calls Matty and tells her that James is trapped. Desi returns with a fire ax, hacks open the lock panel, plants a flash-bang inside, and detonates it with another axe-swing. The blast knocks open the door and stuns Desi, and one agent gets her out while the other one and Riley get James out.
Mac chases Mason to the roof where a helicopter is waiting. He gets in and they lift off just before Mac can stop them, and Mason yells back that it's far from done.
That night at Mac's house, Matty tells Mac that they're searching for Mason. They share a toast with Riley and Wilt. Desi arrives and Mac thanks her for saving James and risking her own life. She says that she only did what she thought Mac would do, and accepts a beer from him. Desi tells him that she's been talking with Jack and he's doing good. He's still tracking Kovacs down so Desi is still protecting the team. Mac says that he's glad and she can tell them in her own time why she owes Jack. Desi agrees and says that they can have dinner and exchange details about their past.
James arrives and Mac hugs his father and invites him to join the team. His father says that he promised to never lie to Mac, and tells Mac that there's more to Mason's story. James says that Mason's son died so that Mac could escape. Mac reminds James that Charlie gave his life to save innocent people like what he did, but what James did violates everything he believes in. He tells James to leave and after a moment, James goes.
Written by Gadfly on May 11, 2019