The following version of this book was used to create the guide: Rundell, Katherine. Impossible Creatures. Alfred A. Knopf, 2024.

Katherine Rundell's Impossible Creatures is a young adult adventure fantasy novel. It is written from the third-person point of view and in the past tense. The following summary abides by a linear mode of explanation for the sake of clarity.

Christopher Forrester went to visit his grandfather Frank Aureate in Scotland when Christopher's dad went away on business. On his first day there, Christopher ventured up onto a hill where Frank forbade him to go. There he found a wounded griffin which he brought back to Frank. Frank explained that there was a magical realm hidden from humans called the Archipelago. The waybetween on the hilltop connected their world to the magical world, but something had happened to open this passageway. Frank and Christopher's late mother Charlotte used to be guardians of the magical realm. There was a tree, called the Glimourie Tree, which was responsible for the glimourie, or the magic that sustained the fantastical creatures living in the Archipelago. Frank and Charlotte used to ensure that nothing happened to the tree. However, in recent days something had gone wrong and the glimourie was fading.

Curious about this world and eager to return the griffin to it, Christopher returned to the waybetween on the hill. Suddenly a girl in a flying coat emerged. She introduced herself as Mal Arvorian and thanked Christopher for saving her pet griffin, Gelifen. She then explained that she needed Christopher's help and asked him to join her in the Archipelago. Christopher accepted and followed her through the waybetween.

In Icthus, Mal's hometown, Mal explained that a murderer had invaded her house and killed her great-aunt, Leonor. She was convinced that the murderer did not simply want to kill her family but was after something bigger. Mal and Christopher tried to escape the murderer and ended up on Fidens Nighthand's boat the Neverfear. When Nighthand heard their story, he agreed to protect the new friends.

Mal, Christopher, Nighthand, and Nighthand's crew agreed that they needed to do something to protect their world. Creatures were dying, the ocean was getting dirtier, and the soils were drying up. They decided to visit the Azurial Senate and appeal to the senators. At the courthouse, Mal and Christopher were excited to discover that a marine biologist named Irian Guinne was appealing to the court for the same reason. However, the senators dismissed these concerns and threw Mal, Christopher, and Irian out.

The companions reconvened on the Neverfear. They decided to go to the sphinx peninsula to ask the sphinxes for help. The sphinxes informed them that the Immortal being responsible for protecting the Glimourie Tree had forgotten who they were because of a memory-erasing potion the last Immortal being had drunk 100 years prior. The Immortal typically protected the glimourie but could not do so now that they did not know who they were. Since then, a wicked man had invaded the maze that housed the tree and was currently trying to take its power so he could rule the Archipelago.

That evening, Christopher realized that Mal was the real Immortal. Nighthand and Irian agreed. They urged Mal to accept her calling and drink a memory potion so she could remember who she was and how to find and save the tree. Afraid, Mal refused. That night, she ran away from the boat, begging Christopher to come with her. In the dark streets, the murderer returned and tried attacking them. Nighthand appeared and saved them, but Gelifen died in the scuffle. After burying the griffin, Mal decided she would drink the potion. She realized it was up to her to save the other creatures like Gelifen.

The friends invoked the help of a centaur to make the memory potion. They traveled to Tār and asked the dryads for help firing and drinking the potion. Afterward, Mal gained access to all of the world's knowledge and remembered where the tree was. Together, she and Christopher located the cave that housed the maze where the tree was located. They wound their way through the labyrinth. At the center, they discovered that a villain called Francesco Sforza was trying to merge with the tree to absorb its powers. The friends yanked him from the maze and flew him out with Mal's flying coat. It was then that Mal told Christopher everything she had learned about the world's pain and beauty and begged him to spread the message. Afterward, she flew Sforza into the sun to destroy him.

Shortly thereafter, Mal's immortal soul was reborn in the body of another child. Christopher and the magical world mourned her death but celebrated her triumph and resurrection. Christopher returned to Scotland and told his family about his adventures, rejoicing that he and his friends had saved the Archipelago.