The grand event for elementary schoolers is finally here--Summer Break! In preparation for the long-awaited trip to the beach, Illya and the others go with their classmates to pick out new swimsuits and carry on in front of the skimpy clothing♪ While they're out, Kuro runs out of mana and calls Illya away to an empty stairway platform. But Mimi catches sight of them in the act?!

Summer! Swimsuits! The beach! Illya and the others enjoy the beach to fullest, playing along the shore and celebrating their birthday in a beach hut♪ But the popsicle seller they encounter there turns out to be the fierce foe they fought a hard pressed battle against... And to make matters worse, Ruby adds a Love Potion that makes Shirou go crazy! Will Illya's desires come to fruition in an unexpected way?!

When they were buying swimsuits in the shopping mall, Mimi caught sight of Illya "resupplying" Kuro. Ever since that day though, she's been unable to stop herself from imagining naughty things. Then, after a few words from Suzuka, Mimi awakens! Illya and Kuro are forced to face the new rose-colored tone Mimi's mind has taken.

After Illya recieves tickets from Luvia, she heads to a theme park with all her friends! As they play to their hearts content, they end up separated from their guardian, Iri. While looking for Iri, they encounter the nurse, Caren-sensei?! Meanwhile, Illya realizes she's lost her precious bracelet charm...

Illya's enjoyable summer isn't over! After changing into a yukata, she joins the usual gang and heads to a local festival. As Illya enjoys yo-yo fishing and lottery drawings at the festival, she realizes something dire--she was supposed to wear underwear beneath her yukata! A great tragedy befalls upon her!

Iri and Liz enjoy playing with water in a vinyl pool in their yard. Then the two of them send Illya and Kuro for a dip too! While they enjoy the approaching end of summer, they recieve notice that Luvia's holding a party to celebrate the reconstruction of her mansion. That was also the signal for them to begin the operation to retrieve the eighth card. The plan will be carried out tonight at midnight!

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.