Plot

Mac and Riley continue their infiltration of Codex, and Gwen is torn in her loyalties. Russ and Desi both suspect Mac.

With Mac and Riley undercover with Codex, hoping to stop the organization from within, their mission is interrupted when Russ, Desi and the Phoenix team move to prevent Codex from detonating a weapon of mass destruction, with Mac and Riley caught in the cross-hairs. Also, the team discovers Russ' real reason for purchasing the Phoenix Foundation.—brahamdali

At the underground compound, Mac meets without Gwen the CODEX boss Leland, trying to answer as he wants on history since the Black Death. Mac and Riley's transport assignment allows Mac, though they are kept in the dark about the group's plans, to deduce it's a nuclear device. Along the way, they are arrested by Russ and Desi at gunpoint, but Mac convinces Matty and ultimately Taylor he's still on their side, but Leland shows footage of that meeting to Gwen and orders Mac and Riley killed, with Gwen's team, at their fake destination. Mac gets Gwen to help him save the world, figuring out that the nuke could trigger the super-volcano under Yellowstone National Park by blowing up a dam and forcing water down a thousand-mile drilled passage. They can't keep the bomb from exploding, only disable the nuclear reaction, but the timing device is destroyed in a scuffle, so Gwen volunteers to trigger Mac's device manually, sacrificing herself. Reunited with The Phoenix, Mac arranges a Senate hearing to present his mother's climate change research.—KGF Vissers

In North California in 1990, Gwen drives down the truck at top speed to the hospital, swerving to avoid traffic. Ellen is with her and says that sent Plan 47 to some friends DC. Gwen points out that Plan 47 is a little dangerous to give the government, but Ellen insists that they need to do something. Ellen goes into labor pains and worries that they'll never get to the hospital. They're forced to pull over and deliver the baby. Ellen tells her new son Mac that he deserves the world and the world deserve him, and names her son after a nearby billboard for Angus beef.

In the present in Codex's underground bunker, Gwen tells Mac and Riley that they used Shiva to underground city's bunker. Guards call Gwen away, and Riley and Mac secretly discuss Mac's plan to find out what codex is up to. Mac figures that there's still a spark of humanity in his aunt, but Riley isn't convinced and warns Mac that he feels connected to her because she knew her mom.

One of the guards comes over with Gwen, and the guard asks Riley to go with him to the tech room while Gwen leads Mac off. The two agents reluctantly agree, and Gwen takes Mac to Leland. She says that Leland paid off the CIA when they came to kill her, and then to fake her death. Gwen tells Mac that Leland is Codex among others. Leland helped resurrect Plan 47, Gwen tells Mac that Leland needs to believe in Mac as well, and doesn't say what will happen to Mac if Leland doesn't.

Ramon meets Mac and Gwen coming down and says that he'll take Mac there while Gwen follows Leland's orders to get her team together. Mac goes with Ramon to a greenhouse where Leland is working with a plant. Leland mentions the Black Plague and how it led to the Renaissance... and then the Codex. He explains that a people go together to protect the world from plague, famine, and dumbasses in government. Amazing science has just come out of the Middle East, but Westerners needed a push so Codex financed the Enlightenment. Then the Industrial revolution came along and Ferdinand was a member of Codex.

There's a J.M.W. Turner painting (Snow Storm: Hannibal and his Army Crossing the Alps) of a sunset on the wall, and Leland explains that Turner was influenced by the 1814 eruption of Mount Tambora. It spewed volcanic particles up into the air, cooling the planet down. Leland tells mac that now things have gotten out of hands because there are too many human. Mac asks what he can do to help, and Leland points out that they have to kill the weakest to save the strongest. Mac provides the answer, and Leland congratulates him and dismisses him.

As Mac goes, Leland tells Mac that years ago Russ answered the question the same way, but he refused Codex's proposal. Mac says that Russ is weak, and Ramon leads Mac away.

Ion the tech room, Gwen tells the people that it's time to go work. Everyone applauds as Mac comes in and goes to Riley. Gwen says that once they're on the road, their tablets will provide further instructions for security. Riley asks Mac if he thinks he can change Gwen's mind, and Mac says that their best plan is to find out what Codex is planning. Gwen comes over, apologizes for the secrecy, explains that Leland does things to keep their veil of secrecy, and hugs Mac.

The teams drive off and Riley tries to hack the tablets without success. Mac has noticed the boxes of dosimeters in the back, and Riley says that she an equation on a white board. Her fellow agent realizes that it's an equation for nuclear reaction, and they wonder if Leland plans to start World War III. When the front tires blows out, Mac pulls over and gets out with Riley, and Russ and Desi approach them, guns drawn.

Mac tells Russ that they're trying to save everyone. Matty and Bozer are watching by remote, and Matty tells everyone to calm down. Mac insists that they're trying to infiltrate Codex, but Russ doesn't believe him. He says that Codex has a nuke, and Bozer tells Matty that Mac is telling the truth. Matty tells Russ to stay down. When Russ refuses, Mac tells Matty that Codex tried to recruit Russ. Matty tells Desi to put her weapon down and put her on speaker. Desi does and Matty orders Russ to stay down.

Russ insists that he isn't compromised, and explains that three years ago Codex tried to recruit him. Codex offered him a large sum to deliver the LA bioweapon to them. Russ refused and Codex took five of his teammates out with an IED. Mac offers his condolences, but says that it doesn't justify Russ buying Phoenix to get his revenge. Russ admits that he can't read Mac and didn't know what he was doing, and Mac tells him that he is trying to stop Russ from killing innocent people.

Matty orders Mac and Riley back undercover while Russ and Desi secretly follow them. Riley gets the coordinates on the tablet and warns that there's one problem: the flat tire. Mac fixes it while Desi watches him. Desi finally tells Mac that she finally broke down and trusted him, and then he pushed her away. Mac says that Desi is the last person he wanted to hurt, but Desi doesn't want to hear his explanation.

Mac and Riley get into their SUV, and Mac tells Riley that things aren't okay.

Desi and Russ approach the coordinates and find a warehouse in the middle of the wood. Mac and Riley go in and find the other Codex teams there. Amber tells them that they're waiting for instructions.

Gwen is in the Codex control room. Leland comes in and orders everyone else out except Gwen. He then tells her that he's always been in awe of her mind and her loyalty, and even her justifiable anger. He takes her hand, and Gwen pats it and asks what is happening. Leland tells her that he has to bring her pain, and Mac is still a Phoenix Man. He shows her satellite photography of Mac and Riley with Russ and Desi, and tells her that now is the hard part for as he hands her a tablet.

A helicopter approaches the Codex warehouse. Desi identifies it as an attack helicopter, and Mac figures that it's a trap and tells everyone to get out before they blow up the warehouse. Ramon is piloting the helicopter, and fires missiles at the warehouse. Most of the people take cover behind metal sheet. In the control room, Leland tells Gwen that Mac and Riley were compromised, and Mac compromised Gwen.

Russ and Desi can't get in because the fire is too big. Mac and Riley emerge from cover and manage to get out. Desi and Russ help them get away, and Russ reports to Matty that they got their fellow agents.

Leland tells Gwen that he and Codex did what needed to be done, and Gwen wonders what she's going to tell Scarlett's sister. He tells her that she should tell Ella that Scarlett died for the greater good. Leland assures Gwen that he and the council trust her, and Gwen knows the truth that some are worth more than others. After a moment, Gwen nods in agreement.

Back at the SUV, Mac says that he's not giving up on Gwen. Matty and Bozer spot a convoy leaving the Codex ranch house. Gwen is in one of the trucks, and Russ and others pull up and Russ fires at the vehicles. Mac climbs out on the hood and tells Desi to pull up closer. He jumps onto the lead truck, unfastens the back, and gets inside. There is a missile, and he identifies it with Bozer's help, and spots a banner with a Yellowstone logo on it.

Gwen comes in and says that it wasn't her but Leland, and insists the ends justify the mean. Mac asks if she's trying to convince herself or him, and says that Ellen wouldn't have wanted the deaths of millions of people. Gwen tells him that no one wants it but it's necessary, but Mac doesn't believe it and says that they just have to give people a chance. He asks Gwen to trust just him, but Gwen doesn't believe his claim that one person can change things.

Gwen hesitates, and Ramon comes in and trains his gun on Mac. He figures that Gwen isn't handling it. Desi slams into the back of the truck, and Mac jumps Ramon. Ramon kicks him out the back, and Mac holds onto the door. Gwen extends her hand to his, and Mac takes it and says that he believes in her . He then lets go and falls off the truck onto the bridge they're passing. Desi pulls over and goes to the edge, and finds Mac handing from the edge.

Once the Phoenix team reunited, Mac tells them about the logo. He points out that's what under Yellowstone is a supervolcano, and Codex is going to use the nuke to set it off.

As Gwen drives the lead truck to Yellowstone, Ramon tells her that he did her a favor and Leland would have had him kill her. Gwen isn't impressed.

As the Phoenix field team drive to Yellowstone, Mac explains that Phoenix plans to release the trapped gas in the supervolcano. Summer would possible disappear as the resulting ash cloud would cover the western hemisphere. Matty hasn't been able to spot the Codex convoy via satellite, but Mac put his phone on the truck so they can track it.

Come sunrise, the team track the convoy to a dam but only spot one truck. Matty and Bozer confirm that Codex set up a dummy corporation to repair the dam, and they've been drilling ever since. Mac brings up a map of the dam and finds a side cent from the dam leading to the supervolcano. He figures that Codex plans to floor the volcano with the water behind the dam, setting off the explosion. Mac admits that he can't disarm the nuke, but he wants to build a shaped charge and put it on the nuke to stop the chain reaction. He leads the others off with him.

Desi takes out one of the Codex guards, and Mac and the others go into the building he's at and find the master control room. There's no sign of the nukes on the feeds, and Russ, Desi, and Mac split up to search the place and find the nuke. Mac finds the materials he needs to create a shaped charge, and tells Desi over the comms how he plans to disrupt the nuke.

Riley spots the Codex operative leave, and Mac figures that they're leaving because they set off the nuke... and he's found it. As Russ heads for Mac, Gwen intercepts him and tells him to wait. Russ draws his gun and she attacks him. They fight and fall down a flight of stairs in their struggle.

Ramon finds Mac, and Mac disarms him and they fight.

Russ grabs his gun, and Gwen says that Mac showed her a better way and she plans to help him.

Ramon knock down Mac, grabs his gun, and aims it at Mac. A gunshot rings out... and Ramon drops, Gwen having shot him. Mac and Gwen go to the bomb, and Mac explains his plan. Gwen realizes what he plans to do, but the time delay fuse was broken in the struggle. He tells Gwen that he'll have to set it off manually.

Desi and Riley find Russ, and Mac tells them to get as far away as possible. Gwen tells Mac to go and she'll set off the shaped charge. Mac refuses at first, but Gwen asks him to do it for Ellen and he goes. Aunt and nephew share a hug and Gwen tells Mac that he deserves the world and the world deserves him. Mac runs off, and Gwen goes to the bomb.

The others get out, and the charge goes off behind them, destroying the nuke. Mac runs out of the smoke, and Desi and Riley go to get him. Russ tells Mac that he did well.

Later at the Phoenix Building, Bozer tells Mac that they're sorry about his aunt. As for Codex, Leland took the rest of the organization into hiding. Russ gives them Phoenix badges and welcomes them back. His leg was broken in his struggle with Gwen, and he tells Mac that he'll have to take over. Mac tells the team that he knows just where to begin on their new jobs.

Later at the Senate building in DC, Mac prepares to go in for a committee meeting with several binders. Desi checks his tie and says that later they'll talk about their relationship. An aide calls Mac in, and Mac tells the Senators that the planet is in trouble and they must apply and do, quoting Da Vinci. He opens the binders and starts discussing the problems.