Lexa: You're the one who burned three hundred of my warriors alive. Clarke: You're the one who sent them there to kill us.
— Lexa and Clarke meeting for the first time
Long Into an Abyss is the seventh episode of the second season of The 100. It is the twentieth episode of the series overall.
Abby is determined to find a way to protect her people from a coming attack by the Grounders, even if it means heading into unknown territory and leaving some captives behind. A worried Bellamy convinces Clarke to go with him to the dropship, where he and Octavia reveal a dangerous secret. Clarke comes up with a bold new plan to stop the Grounders' attack, leading Abby to a desperate decision. Meanwhile, at Mount Weather, Jasper and Monty begin to question the motives of their new friends, while Dr. Lorelei Tsing conducts a deadly experiment to determine how to make use of the captives.
Keenan Mykulak awakens to find herself outside Mount Weather; however, she is not burning from the radiation. She is being monitored on a camera by Dr. Lorelei Tsing and Cage Wallace. At first, she appears to be doing well, but after four minutes and ten seconds have passed, her skin starts blistering and she begs to be let back inside. Dr. Tsing persuades Cage to leave her out there to die so they can get the full results of their experiment as well as prevent Keenan from telling anyone what they are up to.
Abby and Jaha are discussing their options of what to do about the Commander's ultimatum. Abby wants to rescue the kids in Mount Weather but Jaha wants to flee. Major Byrne interrupts and tells Abby that the rest of the Sky People are getting restless and need to hear a word from their Chancellor.
Abby tells everyone at Camp Jaha that, yes, the Grounders have threatened them but they haven't made any decisions yet. Jaha tells everyone to pack and be ready to leave if they need to. He will lead them across the Dead Zone and to the City of Light. Clarke interrupts and asks about the Delinquents still trapped in Mount Weather. Abby tells everyone that no decision has been made yet and they should all report to their station supervisors. As the meeting is breaking up, Bellamy grabs Clarke and tells her she needs to bring a med kit and go with him to the dropship. When Clarke arrives at the dropship, she sees Octavia and a chained-up Lincoln, who had been turned into a Reaper by Cage.
In the Mount Weather dorms, the Delinquents are discussing their dire situation. Monty says they can't keep waiting for Clarke to come rescue them. Jasper agrees and says it is time for them to be the criminals they are and find out the truth.
Back at the dropship, Clarke inspects Lincoln and discovers needle marks on his neck. Lincoln manages to break free and begins attacking everyone until Octavia is able to knock him out again.
David Miller tells Abby that he is not leaving Camp Jaha without his son. Abby tells him to pack his things and report to Major Byrne because she wants everyone out of camp by dawn.
Dr. Tsing is reviewing Keenan's blood results and discovers that the Mountain Men might be able to achieve full immunity if they go to the source of the blood: bone marrow. There are 382 residents in Mount Weather and only 47 Delinquents, meaning that after eight bone marrow operations each, the Delinquents would be dead. She tells Cage that he needs to convince Dante Wallace to move forward with the plan.
Lincoln has been restrained again as Clarke tries to remove the bullet from his leg where Octavia had shot him. Octavia tries to give Lincoln some water and he thrashes at her, upsetting her. She goes outside to get more water and spots a white flower in the tree, the same flower that Lincoln used to leave for her. Nyko appears and warns her that the Commander is on her way and Octavia needs to leave. They then hear Lincoln in the dropship and Octavia brings Nyko to him. Lincoln begins seizing and Nyko kneels down with a vial to give to Lincoln. As he is holding the medicine over Lincoln's lips, he says, "Yu gonplei ste odon" ("Your fight is over"). Clarke recognizes the words from when Anya had said them and stops him from mercy-killing Lincoln. Nyko tells her that death is the only way just as Finn appears to warn them that everyone in Camp Jaha is about to leave. When Nyko sees Finn, he attacks him and strangles him for slaughtering his village. Clarke uses an Ark Guard's shock baton to knock Nyko off Finn just as Octavia realizes that Lincoln has stopped breathing. Nyko watches in awe as Clarke gives Lincoln chest compressions and gets him to breathe again. Clarke realizes that the Grounders have been trying to bring Reapers back but have never been able to do so and she can use that to her advantage to stop the attack.
Clarke goes down to the second level of the drop ship where Finn has been waiting alone. Finn tells her that even if Lincoln lives, he won't be the same. Clarke tells Finn that both he and Lincoln are savable.
Back at Mount Weather, Monty, Harper, Jasper, and Miller break into Dante's office and snoop around.
In Camp Jaha, everyone is preparing to evacuate when Clarke arrives and tells Abby that she knows how to stop the Grounder threat. In the distance, lights start to appear in the darkness of the night and everyone realizes the Grounder army has arrived. Jaha tries to take control of the situation and force the Exodus but Abby stands her ground. Jaha tells David Miller and Byrne to arrest Abby but they only listen when Abby tells them to put Jaha in the stockade. Abby then decides to head back to the drop ship to save Lincoln while Clarke goes to talk to the Commander.
While in Dante's office, Monty, Jasper, Miller, and Harper discover engineering schematics for Mount Weather as well as pictures of Camp Jaha.
Clarke approaches the Grounder camp outside Camp Jaha and is stopped by Gustus who warns her he'll slit her throat at the slightest provocation. Clarke enters the Commander's tent and meets Commander Lexa who is seated on a throne with Indra on one side. Clarke tells Lexa about Mount Weather and about how she and Anya escaped together. She presents Lexa with a braid from Anya's hair. Clarke lies and claims that they can turn the Reapers back into Grounders because she has done it with Lincoln. Lexa demands Clarke to prove it.
Outside Mount Weather, Cage and Dante are in hazmat suits when Cage removes his mask and tells his father to do the same. The Delinquents' blood has given them temporary immunity and Cage wants his father's approval to proceed with the bone marrow. Dante refuses, telling Cage that they have already corrupted their legacy as the history keepers by what they have done to the Grounders and he will not go further.
Abby arrives at the dropship and gives Lincoln a shot to bring down his fever when he starts convulsing again. He stops breathing and Abby begins chest compressions. Clarke is leading Lexa and her guard through the carnage outside the dropship while Abby is trying to get Lincoln breathing again. Abby finally gives up and says that Lincoln is gone. Octavia refuses to believe her and continues chest compressions. Clarke and everyone else enters at that moment and they are at a standstill with everyone going for their weapons. Abby picks up the shock baton and holds it in front of her before using it as a defibrillator on Lincoln, shocking him twice before he starts breathing again, much to the Grounders' surprise and the Sky Peoples' relief.
In the dorms, Miller and Monty are discussing the possibility of their parents being on Earth since they were able to see Alpha Station had made it. Jasper comes up and tells them he has not seen Harper. In the Harvest Chamber, Harper is face-down on a surgery table as Dr. Tsing cuts into her hip with a scalpel. Harper begs Cage to let her go home. Dr. Tsing begins to drill into her for bone marrow and she screams in agony. Meanwhile, Dante, in his office, ponders the yellow flower picked from outside now sitting underneath a glass lid on his desk.
Clarke is back with Lexa in her tent outside Camp Jaha. Lexa tells Clarke that she is willing to have a truce as soon as Finn dies.
Cage Wallace: "The blood of The 47 could be our permanent solution; that's what you said."
Dr. Lorelei Tsing: "Science takes time, Cage. There are dead ends and blind alleys on the path to every breakthrough."
Cage Wallace: "You call that a blind alley? She's one of us."
Dr. Lorelei Tsing: "No, of course not. I call that sacrifice...for the greater good."
Jaha: "You weren't there Abby, I saw them. Warriors trained since childhood to fight and die for their cause. And as we speak they're marching on this Camp. And trust me when I tell you the right choice is to live so that we can come back and fight another day."
Clarke: "I knew Mount Weather controlled the Reapers. I had no idea they were creating them."
Bellamy: "If they can do that to Lincoln, what are they doing to our friends?"
Dante: "Don't play with me son. What will it take to stay?"