At Franklin's grandmother Mamie's house, the Turtle family gathers to welcome Franklin's paternal aunt, Lucy, an archaeologist, who arrives with her goddaughter, Samantha Camille Donda along. Franklin and Sam clash from the start, their playful boasting about age setting a wary tone between them, as they try to find common ground and enjoy the visit.
Among Lucy's belongings, Franklin discovers a hand-drawn map of Turtle Lake, and he immediately recognizes that it was drawn by Granny. The map sparks curiosity in the kids, and at Franklin and Sam's urging, Granny opens up about her own past. She explains that as a child she lived at Turtle Lake with her parents, and one day she found a secret place. She buried a box of valuables inside a painted tin, then drew a map to the location, but never had the chance to open it again. A forest fire soon after destroyed their home and claimed her parents, forcing her to live with her aunt's family. Since that devastating day, she has not returned to Turtle Lake.
Granny's health begins to fail, and the family checks on her with care. When she mentions the buried box, Franklin believes it could be the key to helping her recover. With Lucy's guidance, Franklin and Sam, along with Franklin's friends Bear, Beaver, and Snail, set off for Turtle Lake to search the ruins of Granny's old home. They come up empty-handed at first, unable to locate the box.
A local turtle woman appears and shows them a turtle talisman—similar to the one Granny kept in her box. She explains that the talisman carries a strong psychic power, which convinces Franklin that the talisman might be the true reason Granny wanted them to find the box. The next day, they meet Little Crow, an orphan bird, who leads them to the box's location, only to discover someone has already dug it up. Little Crow then kidnaps Snail to play with his falcon chick friends. Franklin and Sam volunteer to rescue Snail, and after acknowledging their earlier clashes, they learn to work as a team. They reach the mountaintop where the falcon nest lies, and Falcon Mama, having seen her chicks' antics, frees Snail and decides to adopt Little Crow into her family, convinced of his good intentions. This gesture renews the group's resolve to continue their quest.
Following the turtle woman's guidance that “the treasure is under the rainbow,” they travel to a mountain pass and wait for the rain. They enter a cave where they meet a grizzly bear named Grizzly. He reveals that he found the box long ago with a metal detector and added it to his collection, but he cannot remember where he put it. Eventually, they locate the box beneath a crystal chandelier that splits sunlight into a rainbow. Grizzly offers to help them cross the river with his canoe, and the group heads home with the box in hand.
Back home, they present the turtle talisman to Granny, but it appears not to affect her right away. However, when they reveal the other contents of the box, including a photograph of Granny with her parents, she slowly begins to recover, realizing she has finally regained the last memento she had left of her family. The Turtle family celebrates her gradual return to health and happiness.
That evening, Lucy announces she will stay in Woodland for a while to be with her family, while Franklin and Sam say their goodbyes, hoping to meet again someday. Franklin gives Sam the Turtle Lake map so she can always remember their adventure, and Sam returns the gesture with a kiss.
