Koichi Kizakura recalls his reasons for staying with Kyoko Kirigiri, who works to uncover the truth behind Sonosuke Izayoi's murder. But just being at the crime scene is dangerous enough itself. Meanwhile, Byakuya Togami looks for a way to break the Future Foundation out of the building, but certain obstacles prevent him from reaching that goal.
A flashback is shown of Koichi and the former Physical Education Teacher of Hope's Peak's Academy, working together to rescue Class 78th from Junko Enoshima's Mutual Killing. The teacher is killed in battle with only her helmet remaining.
Yasuhiro continues to be attacked outside and is saved by Byakuya. Inside, Makoto and Komaru continue to catch up, much to Toko's disgust. Makoto comments on how Komaru has grown up, and Komaru reminds Makoto of his promise to come and get her. After the transmission ends, the Miaya robot shakes violently and Makoto and Aoi rush out of the room only to come face to face with Kyosuke. Miaya bursts through the wall, and scans Kyosuke as he charges towards the robot and chops its head off. The robot puts its head back and morphs the wheelchair into a type of battle armor. Makoto and Aoi make their escape while Miaya holds off Kyosuke.
In the library, Kyoko continues to gather more clues as Koichi gathers all of the pressure traps Sonosuke placed around the room. Koichi calls Ruruka out on some rumors he has heard about her planning to form her own faction within the Future Foundation. Juzo bursts into the room and attacks everyone. He goes after Kyoko with a knife, but Koichi distracts him. Koichi baits Juzo into attacking him, and then triggers a trap, which sends a spear through Juzo, and pins him to the wall. Koichi mentions how he has noticed that Juzo, the Ultimate Boxer, has been refraining from punching anyone and correctly deduces that his forbidden action is that he cannot punch. To level the playing field, Koichi admits that his forbidden action is that he cannot open his left hand and must keep it clenched for the duration of the game. Ruruka activates Sonosuke's traps and blows the floor out from under Kyoko. Koichi disobeys his forbidden action and opens his left hand to catch Kyoko. As he is being poisoned, Koichi uses the last of his strength to haul Kyoko to safety, and laments how he kept his promise to Jin as he falls into the pit.
Ryota calls Ruruka out for trying to kill Kyoko, and Ruruka says she wants to kill them all. Ruruka forces one of her "special" sweets into Juzo's mouth, brainwashing him to into desiring her sweets. Ruruka promises Juzo all the sweets he can eat if he kills everyone. Juzo frees himself from the wall, and pulls the spear from his shoulder. The pain jolts him back to his senses, and Kyoko explains that Ruruka killed Sonosuke by forcing her sweets into his mouth, which triggered his forbidden action. She knocks over the bookshelf, and reveals the secret exit. Kyoko tells Ruruka her forbidden action is that she cannot allow anyone to leave. Ruruka denies it, but then confirms Kyoko's suspicion when she tries to stop Juzo from opening the door. Kyoko and Ryota leave, and Ryota asks why Koichi sacrificed himself to save Kyoko. Kyoko says she has theories, but has no evidence to hand them up. Ryota notes the coldness in her response, and Kyoko discards her jacket, declaring that she must be cold as she is a detective. Kyoko then remembers Koichi telling her that he had a friend who hated detectives.
Outside, Byakuya and his men blow up the rubble blocking the entrance. Yasuhiro goes to rush in, but Byakuya stops him and tells him to wait outside. As Byakuya and his men enter the building, one of the men walks too close to an unconscious woman, revealed to be a robot, and an explosion is triggered.
The following are in order of appearance
- Koichi Kizakura
- Unnamed P.E. Teacher (Flashback)
- Yasuhiro Hagakure
- Byakuya Togami
- Makoto Naegi
- Aoi Asahina
- Komaru Naegi
- Toko Fukawa
- Miaya Gekkogahara
- Yuta Asahina (Indirectly Mentioned)
- Usami/Monomi (Voice Only)
- Kyosuke Munakata
- Sonosuke Izayoi (Minor role)
- Kyoko Kirigiri
- Ryota Mitarai
- Ruruka Ando
- Juzo Sakakura
- Junko Enoshima (Flashback)
- Jin Kirigiri (Flashback)
- Chisa Yukizome (Flashback)
- Great Gozu (Flashback)
- The title is a reference to an old English nursery rhyme, "Who Killed Cock Robin".
- Kazutaka Kodaka's favorite in this episode is when Kyoko decided to move forward even after Koichi died before her eyes.