Stella was taken to Grand Professor Islands’ secret lab in Japan for his goal of creating the ultimate life form. The Tyrant’s device which was the crystallization of his soul had been processed into reproductive cells by Islands’ cell magic. He plans to scientifically reincarnate the Tyrant’s soul by implanting those reproductive cells into Stella’s body and have her give birth. To make sure it’s a success, Stella’s personality needed to be erased before the reproductive cells are implanted.
Ikki wakes up in Hagun Academy infirmary after being healed by Shizuku. He tells everyone in the room, which includes Shizuku, Touka, Alice, Kagami, Itsuki and Ouma, that Stella had been abducted by Islands. Because the League and the Union are planning to negotiate a ceasefire, Itsuki only allows a small group which consists of Ikki, Ouma, Shizuku, Touka and Alice to go rescue Stella.
All of the memories and experiences that led Stella to become the Crimson Princess are being replaced with false memory by Abraham’s mind control and Islands’ lab equipments, so that it will lead to Stella losing her personality. While this is happening, Ikki and the others have arrived at Islands’ secret base and fight through the base’s defense. Ikki and Shizuku head towards where Stella is, while Ouma, Touka and Alice stay behind to fight Abraham clones.
Ikki is in his normal size appearance and can use some water techniques due to Shizuku merging with his body. He fights Islands in the room where Stella is. Islands is a water user that can turn his body into mist like Shizuku. Islands’s device is cells and flesh capable of taking form of any object or creature. After absorbing Islands into itself, Darwin has taken the form of a giant monster of flesh and attacks Ikki with many creatures like snakes, tigers, crocodiles and others that emerged from the flesh monster. Islands tells Ikki that he only has 30 minutes to save Stella before her personality is completely erased.
Itsuki has a video conference with Stella’s parents and informed them about what happened. They discuss about getting reinforcement for rescuing Stella. However, Norman Creed teleports to the room Itsuki is in and informs them that Arthur Bright has given a directive that the League will not be involved in rescuing Stella.
Ikki is able to defeat the flesh monster with the help of Shizuku’s ability. Islands then emerged from the collapsed flesh but he has taken the form of a young man that looks like Ikki. Darwin then transforms into a sword made of flesh and bones which Islands uses it to fight Ikki with swordsmanship. Islands dedication to obtain all kinds of knowledge has made him capable of copying other person’s techniques just by looking once. His current body is the perfected form of Ikki and so he is able to perform Ikki’s techniques better. But Ikki is assisted by Shizuku who destroys Islands’ body in the end and they free Stella.
Touka, Ouma and Alice arrive in the room after defeating the Abraham clones and they see Ikki fall to the ground after being attacked by Stella who is actually being controlled by Islands. Islands’ brain, which is his core, was removed from his original body long ago and it has grown to the size of a house and hidden in one of his other labs. He was controlling his temporary body by sending out brain waves and used Darwin as a receptor. He says that Stella no longer exists in her body and he’s controlling the empty body through the Darwin cells transplanted into the body.
Using Stella’s body, Islands attacks everyone in the room. Everyone is down on the ground including Shizuku who was already outside of Ikki’s body earlier. Seeing Stella’s body about to finish off Shizuku causes Ikki to believe Stella is gone. Thinking that his future with Stella is now gone, Ikki undergoes Excessive Awakening. The transformation causes his child body to grow back to its original size though no longer looked human, and his magic which was used up earlier has increased. Ikki is able to push back Islands even after Stella’s body has undergone Excessive Awakening too. As Ikki is about to deliver a killing blow, Edelweiss appears and stops him.
Edelweiss earlier brought the injured Nangou to League HQ, and then went with Creed to meet Itsuki who was in video conference with Stella’s parents. Edelweiss acts on her own justice and she offered to help rescue Stella. Bright doesn’t want the League to be involved in rescuing Stella so he allowed Edelweiss who is not loyal to the League to go as reinforcement.
Edelweiss and then the injured Ouma scold Ikki who undergone Excessive Awakening for fighting this way after giving up on Stella and their future together. Shizuku adds that the fact Islands is taking the risk of using Stella’s body in battle rather than securing it means that Stella’s personality hasn’t been completely erased. As Edelweiss fights Islands, Shizuku uses her last remaining magic to fuse Ikki into Stella’s body during an opening. Edelweiss gets injured when protecting Shizuku from being killed and so she is forced to use her trump card “Sword of Cosmos” which amplifies her power similar to Ikki’s Itto Shura. If Ikki doesn’t get Stella back before “Sword of Cosmos” time limit, Edelweiss will be forced to kill Stella and Ikki to prioritize saving Ouma and the others who are barely alive.
Inside Stella’s mind, Ikki finds an old Stella who went through 70 years of nightmare. Islands who is also inside Stella’s mind explains that this woman who went through a different life is no longer the Stella that Ikki knew and she doesn’t even know him. But Ikki knows the Stella he knows still exists based on her holding Laevateinn tight to her chest. Ikki then launches Oikage at Stella, which causes her to remember fighting Ikki and losing to him before. Stella who refuses to be beaten by the same move again then returns to her original young form and blocks Oikage. After Ikki and Stella embrace in happiness, she burns away all traces of Darwin and Islands inside her body.
After being freed, Stella thanks Edelweiss for her help and asks her if she will come to Vermillion. Stella offers to help remove Edelweiss criminal status and give her protection, but Edelweiss declines the offer. Edelweiss who possesses great power doesn’t want to bring trouble for anyone and prefers being a free sword that can save someone without being constrained. Fu Xiaoli, who was asked by Edelweiss, breaks into the secret lab where Islands’ core is hidden and destroys the giant brain, killing Islands for good. Since Islands is killed by someone from the Union, the ceasefire negotiation between the League and the Union will be easier. Itsuki and Tsukikage later discuss about putting an end to the system of having a necessary evil like Rebellion by revealing the truth to prevent another Islands or Or-Gaule from existing in the future.
In the epilogue, Ikki has recovered and returned to his original form. He’s back in Hagun Academy doing his usual morning training and later encounters Kagami. Ikki and Stella who have become too strong have been banned from entering the Seven Stars Sword Art Festival again, but they will debut in the KOK A-League next year. Kurono is also returning to the A-League. Some of the students like Touka and Kuraudo will be entering Japan’s National League after graduation. Moroboshi will go to national university and become a bureaucrat with Itsuki’s recommendation.
Ikki then goes to Vermillion and meets with Stella who is trying out a wedding dress. They were planning to get married after graduating but Stella’s parents and Ikki’s father insisted that they should get married immediately so that Vermillion could have Ikki as a relative before other countries get close to Ikki. Their wedding will take place in Vermillion Royal Palace later in the weekend so they are making preparations. The story about the Failed Knight ends with a pic of their wedding.