In 1995, during Gamera and Super Gyaos' battle in Tokyo, Ayana waited in her parents' car as they were moving their belongings from their house before evacuating. However, Gamera and Gyaos plummeted to the street nearby and resumed their battle, crashing through Ayana's house and killing both of her parents. Ayana was taken in by a foster family living in a small village in Nara Prefecture. For the next four years, Ayana was constantly tormented by nightmares of her parents' death, featuring a horrific demonic version of Gamera. Ayana came to blame Gamera for her parents' death and longed for revenge. Ayana and her younger brother Satoru were constantly bullied by their classmates, and the only one who stood up for them was a boy named Tatsunari Moribe. One day, when Ayana and her brother were walking home from school, a group of three girls approached them and began to pick on them. They told Ayana they would leave her and her brother alone if she entered the shrine of the Ryuseicho and removed its egg. Ayana begrudgingly agreed and entered the shrine, and picked up the stone egg inside. When Moribe found out about this, he rushed to the shrine and found Ayana, demanding that she set the egg down. He explained that his family have been the guardians of the Ryuseicho, a demon that will supposedly end the world if awakened, in the village for centuries, tasked with ensuring the creature never escaped the shrine. He also stated that the Ryuseicho's egg could not be moved by even the strongest men, remarking how strange it was that Ayana was able to easily lift it.
A few days later, Ayana returned to the shrine and discovered that the stone egg had hatched into a bizarre snail-like creature. Ayana was scared at first, but discovered the creature was friendly towards her. Ayana decided to raise the creature, naming it "Iris" after her dead cat. She claimed that Iris' family was killed by Gamera too, and that she would raise it to one day take revenge against Gamera for her. Ayana found a black bead in the shrine and began to wear it as an amulet. This bead was actually an Atlantean orichalcum bead, similar to the one that bonded Gamera with Asagi Kusanagi, that caused a telepathic link to form between Ayana and Iris. Ayana visited the shrine every day to feed Iris, who ate by absorbing food through spikes on its tentacles. When Moribe learned that the Ryuseicho had hatched, he ran to the shrine to kill it with his family's sacred dagger, but Ayana begged him to let Iris live. Reluctantly, he allowed Iris to live so long as Ayana watched over it and told no one about its existence. As Iris grew, it eventually escaped the shrine and entered the local forests, feeding on the animals. When Ayana found Iris was not in the shrine, she panicked and searched for it before finally finding it in the woods. Ayana said she understood that Iris was lonely, and swore she would never leave it alone again. Iris embraced Ayana with its tentacles and took her to the shrine. There, it formed a pod attached to its body and absorbed Ayana inside of it. Moribe went to the shrine to see Ayana and found her merging with Iris. He used the dagger to cut her free and brought her to his house, where he called the paramedics to take her to a hospital.
Eventually, a woman named Asukara Mito and her acquaintance Kurata Shinya visited Ayana in the hospital. Planning to manipulate Ayana's connection to Iris to kill Gamera, who they believed needed to be killed to save the planet from humanity, they took Ayana from the hospital and brought her to Kyoto. Meanwhile, Iris escaped the shrine and went on a rampage, massacring many citizens of the village, including Ayana's adoptive family. The J.S.D.F. tried to engage Iris, but discovered it had grown to 99 meters in height, allowing it to easily wipe out its attackers. Iris then took flight and headed to Kyoto, intent on merging itself with Ayana. Asagi Kusanagi and her friend Mayumi Nagamine traveled to Kyoto as well to get Ayana out of Asukara and Shinya's hands. While they were at the Kyoto train station, Iris descended in the city and approached Ayana. When Gamera arrived, Ayana ordered Iris to kill him. Iris complied and engaged in battle with Gamera. Asagi begged Ayana to let go of her hatred, but her pleas did not reach her. Eventually, Gamera and Iris tackled each other into the station, with Iris impaling Gamera with its spear arm and seemingly killing him. Iris then approached Ayana. ust then, Moribe arrived and called to Ayana. He tossed the sacred dagger at Iris, which bounced harmlessly off the monster. Iris swatted Moribe aside with its tentacle, then forcefully absorbed Ayana into its chest. While inside of Iris, Ayana witnessed flashbacks of Iris wiping out her village, realizing the horrors she cause to happen. She also saw a flashback to the day her parents were killed, and realized that it was Super Gyaos who was responsible for their deaths and not Gamera. As Ayana floated helplessly inside of Iris' chest, Gamera plunged his hand into it and rescued Ayana. Iris responded by impaling Gamera's hand against a wall with its spear arm. Iris began draining Gamera's blood and prepared to fire copies of his fireballs from its tentacles. Gamera blasted his own hand off with a fireball and used the stump to absorb Iris' fireballs and form a plasma fist, which he plunged into Iris' chest cavity, killing the creature in a fiery explosion. Gamera set Ayana down, after which Asagi and Nagamine desperately tried to revive her. Gamera roared loudly, and Ayana awakened. Moribe ran to Ayana and embraced her, causing her to sob and beg forgiveness for her actions. Ayana watched as Gamera walked out into the city and prepared to face down an incoming swarm of thousands of Hyper Gyaos.