We open “Satan’s Spawn” with a repeat of the ending of the previous episode. Rin is understandably not happy with this, so he charges at Satan, yelling Yukio’s name. The scene pulls back, and we hear Mephisto, discussing the events as they occur. Then we go to the opening credits.
When we get back from the opening credits, we see Shura and Arthur fighting. Shura is trying to get past him and Angel is following orders, refusing to let Shura go help the kids who are currently at ground zero of an apocalypse. (To use a Dungeons and Dragons’ reference, Arthur is clearly Lawful Stupid.) They argue, and we get a flashback to Angel talking to Ernst about his plans to destroy Gehenna. Ernst’s actual orders are so offhand and indifferent they barely qualify as orders. We come back to the present and Arthur is declaring that he’ll follow orders even if they are stupid ones.
Shura decides that fighting with Arthur is stupid, and tells him so. Then she runs past him. Arthur is so surprised at her response that he does not try to stop her this time around.
We switch back to Rin and Satan who have a conversation where Rin questions what Satan is doing inhabiting Yukio’s body. Satan states that he had intended to use one of them as “vessels.” Rin attacks, but is knocked across the roof. Satan further explains that he needs a body in order to fulfill his plans to merge Gehenna with Assiah, and Yukio is apparently the perfect fit. He doesn’t want Rin interfering with his plans, so he gets ready to shoot him.
Fortunately for Rin, he’s distracted by Ryuji, who wants to make good on his promise to beat up Satan. The demon in question is not interested in accepting the challenge and throws a fireball at him. One of the instructors jumps between Ryuji and the attack, and bursts into flames that Izumo immediately puts out. The instructor tells the kids to get out of there, which Ryuji is reluctant to do. (In turn, the kids are quite willing to stand behind Ryuji.) Unfortunately, they get clobbered quite easily. Satan is about to finish them off when Shura turns up and blocks him. She sends the kids off to regroup. The fight between Shura and Satan is fairly brief, and ends with an apparently defeat for Shura, who abruptly turns into snakes.
In the next scene, we see the kids including Rin and Shura, trying to come up with a plan. Shura is discussing the tactics necessary to take out the gate and hopefully Satan, but Rin doesn’t like the plan very much because it assumes that there’s no chance of Yukio surviving. He states that he wants to trust Yukio, and that he believes that Yukio can take control over his body. Rin stalks off, which annoys Ryuji. (This would be because of the continuing issue of “Rin runs off to do things instead of letting people help him.”) Ryuji plans on taking off after Rin and Shura, but is sidetracked by Izumo, who states that they should stay where they are and help the other students. The wounded instructor agrees with this idea, so the kids go off to rescue people from the demons.
We cut briefly to Rin and Shura, who have agreed on a plan of attack and then we switch to Satan, who is watching all of the chaos. He has brief flashback to a conversation he had with Yuri when she was still alive. Apparently, he was inspired to try merging the two worlds by Yuri. (Unfortunately, Yuri did not think very hard about the methods that Satan would attempt to use to implement this plan.) He’s interrupted from his daydream by Shura. She talks to Yukio, but Satan gives the traditional “the host can’t hear you,” speech. Shura of course, does not believe him.
In the next scene, we see some of the ordinary students who are attempting to hide from the monsters. Paku is there, and she is trying to keep everyone calm. The monsters break into the room where they’re hiding, but the students are rescued by Izumo and the other kids.
When we next see Rin, he’s attempting to sneak up on Satan so he can get the drop on him. (The tactical set up here is that Shura is playing the bait, while Rin will be attempting to ambush Satan.) Then we switch to Mephisto, who is calmly drinking tea while he watches the battle. Shura meanwhile, is not doing such a good job at fighting Satan. (Or more importantly, getting him to move away from the gate and into an area where he can be ambushed.) The battle is interrupted by Shiemi, who attempts to do the “talk to the person being controlled and get them to kick the mind-controlling entity out,” thing. It does not work very well, but Shiemi seems able to hold her own, at least for a little while. Satan grabs her by the neck and she loses consciousness.
Rin comes out of hiding and yells at Satan, who seems to be really amused. Satan challenges Rin to a fight, but Rin declares that he refuses to attack his younger brother. Satan mocks him and attacks, but Rin just takes the blows and continues to call Yukio to snap out of it. The scene shifts and we see Yukio in some kind of null-space inside of his own head, then we go back to Rin, who is trying to get Yukio to wake up and fight.
Yukio does wake up, but is not initially able to escape. He is currently trapped in his subconscious with all of his fear of and for his younger brother. (This trap takes the form of multiple clones of himself as a child, all of them trying to convince him that he hates and feels jealousy toward his brother.) Yukio tries to deny his fears, but they are extremely loud and persistent. Yukio manages to break loose from his fears to reach out to Rin, and Rin takes Yukio by the hand.
Unfortunately, Satan is still in control of the body, and shoots Rin. Rin collapses, and Yukio screams. Satan tries to convince Yukio to finish Rin off, but Yukio starts fighting the control. Yukio states that he will kill himself rather than harm Rin, and puts the gun he’s holding to his own head while he argues with Satan.
We get a series of flashbacks where Yukio is remembering events from his childhood, and he declares his intention to continue protecting Rin by killing himself, if necessary. Satan is kind of horrified by this, and hurriedly tries to talk Yukio out of his intent, though for some reason, he’s also babbling about wanting to fulfill his and Yuri’s dream.
Rin ends the suicide threat by punching Yukio in the face. Rin yells at Yukio, and they have a brotherly heart to heart. This results in Satan being evicted from Yukio’s body. Unfortunately, this doesn’t stop Satan for very long. Even more demons begin to come pouring out of the gate, while Satan declares that he hasn’t been beaten yet. Then the scene shifts back to Mephisto who is still spectating. He wonders out loud what Rin and Yukio will do next, and then we go to the closing credits.