Plagued by thoughts of lewdness surrounding Illya and Kuro, Mimi starts putting her thoughts down into story, which soon turns into a gender-swapped boy's love story. This soon catches the attention of her classmate Suzuka Kurihara and her sister, who decide to use her story for a yaoi doujin. Concerned about Mimi's gradual transformation into a fujoshi, Suzuka asks Illya, Miyu, and Kuro for their help. Despite not being fond of Mimi's fascination with BL, Illya learns that it doesn't mean they can't still be friends.

Illya receives tickets to a theme park from Luvia and decides to go with her friends. When they lose track of Irisviel, who gets swept into a special event, they come across Caren, who agrees to be their substitute guardian. Later, Illya discovers the charm on her bracelet has gone missing, so Miyu decides to help her search for it. Taking some advice from Caren, Miyu manages to deduce the charm's location and find it before closing time. Afterwards, Kuro becomes curious about the relationship between Irisviel and Caren.

The girls attend the local summer festival, where Illya shows Miyu how to enjoy herself. Later, Rin and Luvia get into mischief in front of Shiro, while Illya uses her transformation to escape a wardrobe malfunction. Meanwhile, Kuro becomes aware that Miyu is hiding some kind of secret from Illya.

The girls are joined by Bazett, who reveals she had dispelled the curse of shared pain a while back, before entering the Mirror World, where they are confronted by the eighth card, shrouded by a black mist. Following their plan, Luvia restrains the enemy while Rin provides a mean to enhance Illya and Miyu's attack, providing an opening for Kuro to attack. However, the enemy manages to block it with a shield and counterattacks, forcing the girls to retreat while Bazett charges in, only to get stabbed by multiple swords wielded by the enemy. As Bazett fights using the Berserker's power, Illya and Kuro stay behind to give her a chance to strike, but are still unable to defeat it. Barely managing to escape, Bazett informs the others that the enemy possesses a second Archer card, before it suddenly appears in the real world.

Luvia collapses the ceiling onto the enemy so that everyone can escape, but it transforms itself into a craft that starts heading towards the city. Just then, the girls are approached by Caren, revealed to be an observer for the Holy Church, who had placed a bound field around the city to keep anyone from seeing anything, giving the girls an opportunity to rest. As Caren theorises that the creature is targeting the Holy Grail, with Kuro deducing it relates to a different Holy Grail War entirely, the enemy starts heading towards Mt. Enzou, where it uncovers a magic circle. While trying to push them out of it, Illya pushes out a human boy that was inside it.

As Caren experiences backflash from her observation spell, the boy, revealed to be the Heroic Spirit of the eighth card, reveals that Miyu is a princess from a parallel world who was born as a complete version of the Holy Grail. Feeling she can't escape her fate, Miyu gives Sapphire to Illya before being pulled into the vortex, completing the Heroic Spirit's ultimate form. Learning that Miyu is still alive inside the demon's body, Kuro and Bazett hold off the demon while Illya, gathering her resolve to rescue her friend, combines Ruby and Sapphire's power to unleash a powerful new transformation.

Using her new power, which puts her own body at risk, Illya fights back against the demon while Kuro and the others provide support. As the demon brings out his strongest sword, Ea, Illya uses her full power to defeat it and finally rescue Miyu. Following the battle, Illya and her friends get together to finish their summer homework, with Miyu more than happy to help. Meanwhile, Irisviel and Caren discover the crater has disappeared into a parallel world, where the eighth card is recovered by a mysterious pair.