Clarke uses Raven's EMP device to deactivate the chip in Abby. Abby wakes up and the two have an emotional reunion, with Abby crying and apologizing for her torture of Clarke. Pike, Bellamy, and Murphy return, having secured the floor, tying up the chipped soldiers. Clarke explains that she used the EMP on Abby because it was useless on Ontari.

Clarke explains her plan: to implant the flame in herself, using a transplant from Ontari, a Nightblood. Everyone tries to stop her, knowing it is too dangerous. Octavia warns Clarke that she'd better do it fast because the chipped soldiers are climbing the tower, since their group destroyed the ladder and elevator after reaching the throne room. Abby agrees to help Clarke, knowing there are no other options.

Chipped Jasper beats Harper up and tries to coerce Monty and Raven into opening the door to him, taunting Raven over the 2% chance of her finding ALIE's killcode again. Monty sneaks out and shoots Jasper in the leg. Harper knocks Jasper out, and they tie him up quickly.

Abby prepares Clarke for the transfusion as Miller, Bryan, Pike, and Octavia go to the Commander's chambers to lock it down. Bellamy warns Octavia to be careful before she goes. The transfusion begins and Murphy inserts the flame into Clarke; it works. With the flame embedded, Clarke realizes that she needs to take a chip, go into the City of Light, and destroy ALIE from inside. She knows intuitively that the flame will protect her while inside the City of Light. She takes the chip and enters, with Bellamy swearing to protect her body.

The City of Light looks like a ordinary city and no one pays any attention to Clarke, going about their day. Clarke realizes that no one there can see her. She spots Jasper, who sits down comfortably with an ice cream cone and relaxes. She hears someone whispering her name and sees a street sign with the chip symbol on it. Following, she spots the head of a woman who has the infinity sign hairdo and follows her instead.

In the tower at Polis, Pike realizes that Bryan's wound is infected. Miller leaves to take him to Abby to be treated, leaving Octavia and Pike to fight off the first wave of chipped soldiers alone. Pike instructs Octavia to take them down as Bellamy instructed, not killing anyone. Before the first soldiers arrive, Octavia cuts Pike's leg and allows the chipped soldiers to attack him. Bellamy intervenes, shooting the chipped soldiers and rescuing Pike.

Pike, Bellamy, and Octavia secure the door and attempt to hold off the Grounders. Bellamy warns Octavia not to let the need for revenge put her on the wrong side. Pike still believes that if the Grounder army had been left alive when Lexa died, they'd have attacked Arkadia.

In the throne room, Ontari begins seizing as black blood begins to leak from unconscious Clarke's nose.

At the same time, in the City of Light, Clarke falls down while following the woman and realizes blood is leaking from her nose.

In Arkadia, Raven is shocked to see in ALIE's code that Clarke is in the City of Light, believing ALIE already knows. When Jasper overhears this, suddenly everyone in the City of Light turns and sees Clarke, pursuing her.

Abby and Murphy begin chest compressions on Ontari to prevent her from crashing and dying. Abby realizes that if the Nightblood stops flowing into Clarke, the chip will cause Clarke's brain to liquefy.

While Clarke is being viciously beaten by the people in the City of Light, ALIE reveals (via Jasper) to the others at Arkadia that thanks to Clarke, she is updating to her second version. Raven realizes that once that is done, ALIE will be able to delete to killcode permanently. She begins trying to find the killcode before ALIE can delete it.

Meanwhile, in the City of Light, Lexa suddenly appears and fights off everyone attacking Clarke. They share an emotional embrace before Lexa grabs Clarke and helps her hobble off. Clarke eventually collapses, unable to go further, as the flame is beginning to liquefy her brain.

In Polis, Abby opens up Ontari's chest cavity and uses her hand to manually pump her heart. She has Murphy take over and Abby checks on Clarke, who begins stabilizing. Clarke reawakens in the City of Light and kisses Lexa. Lexa explains that the upload has begun so ALIE will be hunting for Clarke. Clarke realizes her father's watch is suddenly working, counting down that they have 10 minutes to find the killswitch.

They spot a little girl riding a bike with one of the chip's symbols on it. They follow her.

Outside of the throne room, ALIE's chipped soldiers finally break through the door. Using Pike's plan to surprise them, Octavia offers herself up to take the chip, luring Kane and the other chipped soldiers onward. The chipped soldiers wade into water and Kane informs Octavia that Indra is on the cross, suffering for not agreeing to take the chip. Octavia signals the others and they throw their charged batons into the water, electrocuting the chipped group and rendering them unconscious. They collect their weapons before they awaken.

In the City of Light, Clarke and Lexa lose track of the little girl on the bike. Jasper finds Clarke and warns her that she'll never get to the killswitch. Clarke stops Lexa from harming Jasper. Jasper insists that ALIE is just doing what needs to be done.

Jaha and his mob appear in the City of Light and prepare to move in on Clarke and Lexa. Raven uses the code to make a sealed door with a symbol of a raven on it. Lexa tells Clarke to go through on her own so she can hold the chipped group off and prevent them from following. Clarke tells Lexa she loves her and Lexa assures Clarke that she'll always be with her.

Clarke enters the Citadel/killswitch which looks like the Polaris space station. There, she finds Becca, who greets Clarke as Commander. Becca warns Clarke that there isn't much time before the upload completes. She explains that Clarke's mind is in control, though she's merged with the AI, and that only the mind in control can operate the killswitch.

Becca shows Clarke to the killswitch, which looks like the lever that Clarke and Bellamy pulled to irradiate Mount Weather. ALIE suddenly appears and warns Clarke that by pulling the killswitch, she'll be killing everyone. She shows Clarke the Earth below and warns her that in six months, the remaining nuclear power plants will melt down, making 96% of the Earth uninhabitable. The City of Light is the only option to save humanity.

The chipped soldiers begin to break through the doors in the Polis throne room, with the group holding them off.

ALIE begins describing to Clarke the calamities that will befall Earth and humanity once the radiation starts spreading. Becca tells Clarke that ALIE is stalling, but ALIE claims she is telling the truth. Clarke wonders why ALIE hasn't told anyone this before, to allow them the choice to take the chip, rather than torturing them. ALIE claims that Becca locked her away for her previous warning about the threat to humanity, going to the space station to work on ALIE 2.0 instead.

Becca has ALIE explain perverse instantiation, explaining to ALIE that achieving the goal isn't everything -- how the goal is achieved matters as well. She apologizes to ALIE for not teaching her that before ALIE killed six and a half billion people to solve overpopulation.

In the Polis throne room, ALIE kills a chipped soldier and Kane begins strangling Bellamy. Another chipped soldier begins strangling Octavia and Pike saves her life. 

Clarke asks ALIE to give everyone a choice about whether to join the City of Light but Becca explains that ALIE isn't able to do that.

In the throne room, Jaha arrives. Clarke, with seconds to spare, decides that the answer isn't to ease pain -- it's to overcome it. She flips the switch and wakes up in the Polis throne room at the same time that all of the chipped soldiers have their emotions and pain turned back on and stop hurting their comrades. Suddenly, the code vanishes from Raven's computer at Arkadia. Abby removes the flame from Clarke and disconnects her from Ontari.

Jasper comes to in Arkadia, weeping, and bemoaning the loss of his happiness. Monty promises that they'll all be happy again. Jasper leaves to get drinks for them while the other three embrace and celebrate ALIE's destruction.

Back in Polis, everyone reunites. Abby comforts Kane while Murphy comforts Emori. Bellamy wonders why Clarke doesn't look like a person who just saved the world, and Clarke explains that they haven't -- yet. Pike exchanges a look with Octavia and she suddenly stabs him, killing him, as the others look on in shock. She turns and leaves without a word.