Jaha is leading Murphy and four others through the Dead Zone as they search for the City of Light. Harris is telling cheesy bar jokes about Grounders, Reapers, Mountain Men, and Arkers when he suddenly explodes, knocking everyone off their feet. Another person in their group scrambles away in terror and she quickly blows up, too. Jaha realizes they've walked into the middle of an unmarked mine field.
In Mount Weather, Emerson enters the Command Center and Cage Wallace asks to be briefed on the remaining 44 Delinquents who have disappeared. Emerson tells Cage that he has people going from room to room looking for them. He also mentions that there are people inside Mount Weather who don't agree morally with what they're doing to the Delinquents. They discuss using the fact that the Delinquents irradiated an entire level, killing ten of their soldiers and Dr. Lorelei Tsing to flush them out. They then discuss the Grounder army stopped just outside The Veil's range and Cage wants to know why.
In the Grounder camp, Clarke is stressing over the battle plans and Lexa tells her she should rest because she is wasting her energy. Clarke worries over whether or not Bellamy will be successful and Lexa tells her that is what it means to be a leader. Lexa tells her she was born to be a leader, just like her.
In Mount Weather, Bellamy discovers his access card has stopped working. Raven asks over the radio if he has found the Acid Fog room yet and he tells her he will call her back once he finds another way in. Garza and another Mount Weather Guard stop Bellamy and report back that they have found the target who is not Lovejoy. Bellamy takes off running and they chase him through the Mountain. Bellamy ducks around a corner and waits for Garza to close in before he knocks Garza out and steals his key card.
Raven and Wick are at Camp Jaha working on a solution to help Bellamy disable the Acid Fog. They discuss the possible components of what makes Acid Fog and how they can go about neutralizing those components.
In the Grounder camp, Clarke approaches Octavia across from a campfire. Octavia confronts Clarke about the bombing of Tondc and asks Clarke to tell her she didn't know it was coming. Clarke tells her she did it to save Bellamy and Octavia tells her that Bellamy would have never agreed to it and would have found another way. Clarke tells Octavia she can't tell anyone and Octavia says she knows; the alliance would break. They are interrupted by Lexa and Ryder, one of her warriors. Ryder tells Octavia to go relieve the southernmost guard position. Octavia tells them Indra expects her to be on a scouting mission with Lincoln. Ryder tells her that Indra will join her there shortly and Octavia walks off. Clarke tells Lexa that Octavia won't say anything and Lexa says she cannot be sure of that. After Clarke leaves, Lexa tells Ryder to kill Octavia.
Bellamy has successfully made his way back to Vincent Vie when a Mount Weather Guard appears. Bellamy pulls a gun on him, but Vincent tells him it is all right. The guard, Lee, tells Bellamy that some of the guards do not agree with Cage either. Bellamy asks after the Delinquents and Vincent tells him they'd been moved to already searched areas because Cage had gone public with what happened on Level 5. Bellamy asks for help getting to the Acid Fog room and Vincent shows him designs of the retrofit areas in Mount Weather and what they have been using to move the Delinquents in secret. Vincent gives him an acetylene torch and shows him the route to take.
Clarke spots Indra seated near a fire and asks her why she's not on watch with Octavia. Indra tells her she doesn't stand watch and Clarke asks where Octavia is. Indra tells her that she's with Lincoln, scouting as planned. Clarke heads off to find Octavia and spots Ryder with an arrow aimed at Octavia. Clarke points her gun at his head and tells him to stand down and come with her and she leads him away from Octavia.
In the Dead Zone, Jaha and his men are waiting for a sandstorm to clear while they sit in the middle of the minefield. Once the sand stops blowing, they see all of the footprints are gone. Jaha spots a glow from the sunlight reflecting off something in the distance and they believe they're getting closer to the City of Light.
Clarke brings Ryder back to Lexa and tells her that Octavia is not a problem. Lexa tells Ryder to stand down and sends him away. Clarke tells Lexa she can't kill anyone she doesn't trust. Lexa says yes, she can. She calls Octavia a threat and Clarke tells her that Octavia wouldn't ever endanger her brother's life. She tells Lexa that she's a liar and really does have feelings, about Gustus, about Costia, and about the people who burned in Tondc. Lexa tells Clarke that she didn't let her burn. Clarke asks for Lexa to trust her that Octavia is not a threat and warns Lexa that if she does anything to Octavia, Clarke will tell everyone about the missile.
In the Dead Zone, Jaha is slowly making his way through the minefield using his cane to detect the mines. Murphy, Craig, and Richards follow behind in Jaha's footsteps as they slowly inch their way closer to the presumed City of Light. Richards gets impatient and takes off running toward it before Jaha tackles him inches away from stepping on another landmine, saving his life.
Back in Mount Weather, Bellamy uses the acetylene torch to cut his way into the Acid Fog room through an air vent.
Wick and Raven are in Camp Jaha, listing all the acids they know and which ones seem the most likely candidates for the Acid Fog solution. Raven wants to take down the machine while Wick wants to neutralize the acid.
Bellamy grabs an emergency fire ax and uses it to bust the door lock to the Acid Fog room and jam the doors from being easily opened. He calls into Raven and begins to describe the Acid Fog room. Wick tells Bellamy to go to the monitor and asks after Monty. Bellamy starts reading off what he sees on the monitor and asks if he can just blow it up. Wick tells him that would let Mount Weather know their defenses are down while Raven tells him he would probably melt his face off.
In the Dead Zone, they've finally made it to the end of the minefield and see the sign warning of said mine field on the other end. They begin climbing the sand dunes, trying to get to the City of Light only to discover it is a giant solar panel farm. Murphy is disgusted that there is nothing there but Jaha refuses to believe that's it.
Wick and Raven continue trying to guide Bellamy through how to disable the Acid Fog while Bellamy reads them all of the menu options. When he gets to "Maintenance and Cleaning," Raven and Wick up with the idea to use the "Passivation" process to neutralize the acid. Bellamy selects it and the machine whirs to life. He watches as the pH rises from 0 (acidic) and climbs to 7 (neutral) before a message flashes that the passivation was successful. Raven and Wick accidentally hug in celebration and Bellamy tells them to send a flare to let Clarke and Lexa know the Acid Fog has been disabled.
Lexa summons Clarke to her tent to tell her that Octavia has nothing to fear from Lexa because Lexa trusts Clarke. Lexa tells her that their ways may be harsh but it is how they survive. Clarke asks if maybe they deserve more out of life than just surviving and Lexa kisses Clarke. Clarke returns the kiss before breaking away, telling Lexa that she is not ready to be with anyone. The Grounders outside raise the alert that a flare has been spotted and they prepare for battle. Lexa tells them to sound the horn and announces they are going to war.
Raven returns to the lab after shooting off the flare and begins kissing Wick. They have sex.
As the Grounder Army marches on Mount Weather, Bellamy is leaving the Acid Fog room when he notices an old manual dial built into the far back on the side of one of the huge barrels of acid. He cleans off the dial and realizes that the pH is still set to 0. The computer was wrong and the acid is not neutralized. He calls Raven on the radio, however, Cage and Emerson had intercepted the signal when they first discovered Bellamy and allowed him to think he had disabled the Acid Fog machine before cutting his communication with Raven. Cage tells them to reset the Acid Fog panel as they wait for the Grounder army to get into range. He tells Emerson to bring him the intruder.
Bellamy hears the Mount Weather Guards banging on the door to the Acid Fog room as they try to get in and he takes off running back toward the air duct he originally climbed through. Garza and five other guards chase after Bellamy and he hides behind one of the flammable oxygen tanks before making a run for the air duct as the Mount Weather Guards shoot at him. Once he is in the air duct, he shoots behind himself at the guards.
Cage tells Emerson to wait 10 more seconds before deploying the Acid Fog because he wants to make sure all of the Grounder army is inside the kill box as the army continues to march closer.
Bellamy runs out of bullets and scurries through the air duct as Emerson begins a countdown on the Acid Fog deployment. One of the guards goes after Bellamy in the air ducts while another guard checks the tanks in the Acid Fog room. When Emerson counts down to one, Cage says, "we're go for Veil." The Acid Fog machine starts to fire up just as one of the guards spots the lit acetylene torch next to the flammable oxygen tank. A massive explosion takes place, blowing out everyone and everything in the Acid Fog room and traveling through the air ducts. It takes out the guard that had followed Bellamy and the force of the explosion reaches Bellamy just as he gets to a ventilation shaft, pushing him out before the fire reaches him. Bellamy smiles with relief that he is still alive.
Emerson tells Cage the Acid Fog did not work and that there was an explosion in the Acid Fog room. Emerson says that the Reapers can't take on a force the size of the Grounder army and Cage gets upset, asking what else they have. Emerson admits they have nothing but the door left.
At Camp Jaha, Raven gets dressed while Wick tells her to stop playing games and figure things out. Raven tells him they have a war to win.
Octavia is marching through the forest with Indra and the other Grounders when Clarke walks up and tries to assign Octavia to the Rear Guard where she will be safe. Octavia tells her she doesn't take orders from Clarke; she takes them from Indra. She breaks off their friendship. Indra tells Clarke Lexa is looking for her and Clarke takes off. Indra and Octavia discuss the Tondc bombing and Indra tells Octavia that Lexa and Clarke didn't do anything; it was the Mountain Men. They sound the battle cry as they continue their march.
In the Dead Zone, Jaha contemplates why all of the solar panels are still fine when the rest of the world is broken. Murphy says he can fix that as he throws a rock at one of the solar panels, breaking some glass. A drone suddenly flies out and pauses in front of them before taking off across the shore of a sea where a boat is conveniently waiting for them. Jaha tells them they are going to follow the drone and all four of them get in the boat and start rowing after the drone.