Based on the Fate/Kaleid Liner Prisma☆Illya 2wei! manga by Hiroshi Hiroyama, which is a spin-off of the Fate/stay night erotic visual novel by Type-Moon.

Waking up to find herself in a parallel version of Fuyuki City, Illyasviel "Illya" von Einzbern is lost and alone. She discovers her home in ruins, with a massive crater lying in the center of her hometown. With snow falling in the middle of summer, confusion consumes the young elementary schooler, who has no knowledge of where her friends or her wand Ruby may be. Making it to the remains of her house, she is suddenly tackled by an amnesiac girl. Wearing a gym uniform as the icy temperature sets in, the mysterious girl has no idea of where she is or why she showed up. However, this stranger, known as Tanaka, apparently has information about the location of Rin Toosaka, Miyu Edelfelt, and the rest of Illya's missing friends. Fleeing from agents of the Ainsworth family⁠—those in control of this parallel realm—where will these two end up, and how will Illya restore the present back to the world she once knew? [Written by MAL Rewrite]

The Holy Grail War is a battle royale among seven magi who serve as Masters. Masters, through the use of the command seals they are given when they enter the war, command Heroic Spirits known as Servants to fight for them in battle. In the Fifth Holy Grail War, Rin Toosaka is among the magi entering the competition. With her Servant, Archer, she hopes to obtain the ultimate prize—the Holy Grail, a magical artifact capable of granting its wielder any wish. One of Rin's classmates, Emiya Shirou, accidentally enters the competition and ends up commanding a Servant of his own known as Saber. As they find themselves facing mutual enemies, Rin and Shirou decide to form a temporary alliance as they challenge their opponents in the Holy Grail War. [Written by MAL Rewrite]

In the midst of the Fifth Holy Grail War, Caster sets her plans into motion, beginning with the capture of Shirou's Servant Saber. With the witch growing ever more powerful, Rin and Archer determine she is a threat that must be dealt with at once. But as the balance of power in the war begins to shift, the Master and Servant find themselves walking separate ways. Meanwhile, despite losing his Servant and stumbling from injuries, Shirou ignores Rin's warning to abandon the battle royale, forcing his way into the fight against Caster. Determined to show his resolve in his will to fight, Shirou's potential to become a protector of the people is put to the test. Amidst the bloodshed and chaos, the motivations of each Master and Servant are slowly revealed as they sacrifice everything in order to arise as the victor and claim the Holy Grail. [Written by MAL Rewrite]