In the heart of a quiet suburban neighborhood, Conor, a young boy home from boarding school for the summer, wakes up one night to mysterious noises emanating from his garden. With curiosity piqued, Conor embarks on an adventure that will change his life forever. His father's frequent travels and his mother's work at a fashion boutique leave Conor with plenty of time to explore and make new friends.
Conor's summer adventures involve playing tennis with Siobhan, the girl next door, and forming a group of friends known as "The Dino Five." They explore the town of Renelagh on their bicycles, attaching playing cards to their spokes for a unique sound. However, the noise in the garden is not the playful laughter of his friends but something much more intriguing.
The source of the noise leads Conor to the garden shed, an old, weathered structure that serves as both a storage space for garden tools and a secret hideout for him and his friends. Curiosity drives Conor to investigate a peculiar hole near the shed, and to his amazement, he discovers a creature living beneath it. This creature, who introduces itself as Bjon, is a troll from a faraway land. Conor and Bjon quickly form an unexpected friendship as Conor begins to bring food to the creature.
Conor introduces Bjon to Siobhan, and their trio becomes inseparable. Siobhan's curiosity and enthusiasm add a new dimension to their adventures, and they embark on a series of journeys that lead them to the discovery of a mysterious map sent by Conor's Uncle Jack in Canada. This treasure map becomes their ticket to even more thrilling escapades, as they navigate forests, rivers, and mountains, solving riddles and overcoming obstacles along the way.
Their adventures eventually bring them to a secluded island with a hidden cave behind a majestic waterfall, where they uncover a treasure trove of ancient artifacts and gemstones. However, their discovery triggers a perilous trap, and they narrowly escape, emerging with an even greater treasure: their unbreakable friendship.
Back in Conor's garden, they examine a mysterious glass globe that mysteriously glows when held. Their adventures have only just begun, and the magic of their newfound friendship lights the way for more exciting journeys in the future.