Haru has had an imaginary friend for his entire life, a boy wearing a crown visiting him whenever he feels down. The only person who has ever taken him seriously is his best friend and crush Kotona.
Kotona one night on her way home from one of Haru's basketball games, she plans to meet Haru at a cafe but she is stabbed by an unknown man. Haru sees this, but not in time. He rushes to help Kotona and his imaginary friend comes to see him.
His imaginary friend tells him to go to the sidewalk, and keeps giving him advice on how to help Kotona. When they get to the side of the street the imaginary friend pushes him into oncoming traffic.
He is then isekai-ed. When he opens his eyes he is in Evermore, his imaginary friend welcomes him to the other world. The imaginary friend who Haru had called Yu for all the years they knew one another introduces himself as Ferdinad.
Haru asks where Kotona is. Ferdinand shrugs and is like "whoops silly me lets go find her" Haru gets pissed at him for not taking care of her, then a girl appears next to Ferdinand with a sword protecting him. This girl looks identical to Kotona. She is bandaged and her wound looks extremely infected.
Ferdinand explains this is Astrid the head guard of Evermore,and Kotona's sould-mate. If Kotona dies she will too, so that's why they gotta find her. He also explains that he and Haru are soul-mates.
Meanwhile Kotona wakes up in the Miasma Marshes. She is imprisoned by Galorath, he is like "omg the head guard of Evermore, right into my trap. Hohohoho" and he isn't green. He's like Mandarin just wearing a mask. He sends a message to Evermore saying that the Black Banner has taken the head guard hostage and she will only be released if the prince surrenders Evermore to him. Haru is like "dude we gotta convince the prince to give Galorath the crown" and Ferdiand explains that he is the prince. Ferdinand takes Haru into the castle and they meet with Gnauss the head mage and Ferdinand's little brother. Haru is confused because he has no siblings. But Ferdinand explains not everything in each world stays in the same dynamic. Ferdinand tells Gnauss to watch the kingdom as he and Haru go save Kotona. Gnuass mocks him and says he always goes on his own for adventures and doesn't even care about his kingdom. Ferdinand waves him off and he and Haru leave.
On the way Haru and Ferdinand bump into some weird looking bandits and they start asking them questions about where they came from and what they are doing. The two of them are both startled by the encounter and while the bandits are talking,
they run away from them, leading them to a cave that bekans them in. Haru advises Ferdinand against going in, but he cannot turn away, so he goes in anyways. The cave was dark but suddenly lights up as he goes closer in, he then sees a little boy
who seems to be no older than 7 or 10. Ferdinand goes up to him but soon finds out it was a bad idea, as behind the boy there is a bunch of people who don't seem to like the idea of him being in there with them. Haru sees what is going on after hearing Ferdinands yell and readies for battle,
only to see a bunch of little kids running at him with toy swords. With these things it is shown that Ferdinand is extremely incompetent without the protection of Astrid. Over time he slowly begins to be more confident and have his own strength.
They make it to the Miasma Marshes but before they can get through they are ambushed by members of the Black Banner Haru is able to fight them away from himself but Ferdinand is taken hostage.
He gets thrown into a cell near Kotona, Kotona is relieved to see who she thinks is her best friend, she is still in pain but her wound is bandaged. The two then plan to escape Ferdinand lying and saying he's Haru.
Haru ends up sneaking into the castle and seeing Galoreth, Galoreth like in the movie tries to convince him this world isn't real so it would be okay if Ferdinand were to die. Since nothing is real. Galoreth says he'll let Kotona go if Ferdinand stays here. Haru is about to accept the deal but its the cliche fake out deal and he pulls a sword on Galoreth and tries to stab him. He misses and his mask comes off revealing its Gnauss. Meanwhile Ferdinand helps Kotona escape and run into Haru and Gnauss fighting. Ferdinand calls out to Haru to leave, distracting him leading to Gnauss shoot him with a spell blasting him away. Kotona limps towards him and tries to help him, Ferdinand runs and hides. While he's hiding a similar scene to the one in the movie with Oliver plays, except instead of it being an alive old man Oliver it's the Conductor. The Conductor gives Ferdinand Mornstar and tells him to use the weapon his father did to save the world as his father had done. Gnauss is about to kill Kotona and haru before Ferdinand rushes behidn him and stabs him with the sword. It doesn't kill him but it hurts him. Haru gets up and also stabd Gnauss. The three of them have an emotional fight scene ending with Ferdinand and Haru winning. Ferdinand kneels over his brothers dead body and cries. Kotona and Haru comfort him before they all leave and return to Evermore. There all the citizens greet them and applaud their efforts. Astrid runs to Yu and kisses him, she is healed and Kotona's wound is beginning to heal as well.
Ferdinand with Mornstar casts 'Gateway' and Kotona and Haru return to their world. The ending scene remains the same with the two of them going up the stairs to the cafe they wanted to go to, but instead they're holding hands instead of just walking with each other.