The events of this film focus around Franklin's Granny falling ill during a visit from Franklin's Aunt Lucy, who is an archaeologist. Aunt Lucy knows of a special talisman that may cure the illness, so she joins Franklin on a quest to find it, along with his friends Bear, Beaver, and Snail. But they are also accompanied by Aunt Lucy's goddaughter, Samantha "Sam", and her personality clashes with Franklin.
The film was released direct-to-video in the United States, by HBO and has had TV airings on Nick Jr. since then. Writer John van Bruggen, stated "this film will take Franklin on a much greater adventure than any of the past Franklin films, including Franklin and the Green Knight." The film was written by John van Bruggen and directed by Dominique Monfrey. It is distributed by Mars Distribution, with StudioCanal controlling all rights worldwide. This film has also been listed under the working titles "Franklin and the Secret Talisman" and "Franklin and Granny's Secret".
The movie opens with black and white and a young female turtle with glasses. The scene then catches on fire and flashes out on Granny Turtle, who wakes from a bad dream, and sighs sadly. The scene then pulls us to Franklin, Beaver and Snail playing pirates, and they were about to go to a tree with an X. Just as they were about head towards there, Bear appears playing the villainous captain and Franklin defeats him. After that, the four went home. Franklin and Snail walk by Franklin's Aunt Lucy's place and notice the door was open. They went inside to see if Aunt Lucy was home, but instead someone in a mask scares them off and they run away. As they run off, the someone in behind the mask reveals to be a female turtle with a pink bonnet on the head. two run, where Franklin's parents and sister Harriet get a message of Lucy, with no post or stamp. When Franklin revels what happened, Jonathan soon gets the answers and they walk around the house, where Lucy appears to be. Lucy was at Granny Turtle's (hers and Jonathan's mother) and promised to invite the turtles over.
When Franklin tells her about the monster at her place, Lucy thinks it's her goddaughter. When the turtles arrive at Granny's place, who is happy to see them here, Lucy is in the backyard, with her goddaughter who reveals to be the turtle with pink bonnet, named Samantha (or "Sam"). The two aren't getting along very and despite trying to be to one and each other, their personality clashes keep getting the best of them. Later the next day, Bear, Beaver, and Snail wait for Franklin to play pirates again, when they see him with a bouquet of flowers. They decide to follow to see what's up. When they arrive, they meet Samantha, who thinks pirate games are for little kids, until Aunt Lucy appears as a pirate. As Sam is still not interested, Lucy decides to show her a map to shows real treasure: Granny's box of memories. Franklin and Sam decided to meet Granny to talk to her about the box. Granny flashes back to when she was just child, revealing to be the turtle we had seen at the beginning of the film. She buried her valuables inside a painted tin box in the ground to keep and remember, but she never opened it again. A few nights after she buried it, her parents let her sleep outside in a tent when a forest fire started and burned her tent...and was about to damage her house. She managed to escape on the lake in a boat when the rain started. The next morning, her house was destroyed, the location of the time capsule was obscured, and her parents were killed. She went to live with other relatives and hadn't been back since.
Interest by her past, Franklin and Samantha decided to check on memories from Jonathan and Lucy's past. Later, the turtles get a call from Lucy that Granny is sick, and they decided to check up. Thinking that the box may be their only hope, Franklin, Sam, Lucy, Beaver, Bear, and Snail decide to head towards to Granny's old ruined home. During the adventure, Sam pulls a dirty trick by placing a rock in Beaver's backpack, causing her to slow down, until she founds out when she looks inside. She is not impressed and laughable at this joke. Suddenly, a flock of butterflies appears and they follow them, where they meet an old wise turtle, who gives an item, in which Granny also has it was imminently given back. Next day, they meet Little Crow, an orphan bird, who shows them the place with box use to be, revealing to be taken. Later, after placing flowers over Granny's house, they head out where Little Crow takes snail to where three baby birds are.
Meanwhile, Bear, Beaver and Lucy try look for the two turtles, who went to looking for Snail, and find an arrow Sam left for them to follow. While trying catch up get, Franklin and Sam's personality clashes happen again, until Little Crow, crying because the chicks wouldn't let him play with Snail (or "Pretty Shell" as he would call him), wanting to keep him to themselves. As they climb to reach to Snail, Samantha's hand slips, causing her to fall, but is saved by Franklin. They decided to rest rock that's safer not to fall. While waiting, the two turtles apologize for what they said and did to each other, admitting that their personality clashes had gotten the best of them. They soon reach the top and try to save snail, but chicks refuse to let them take Snail. Little Crow manages to save Snail, but the chicks attack, causing him to throw Snail. Sam saves him but they all fall when the mother of the chicks appear and save them just in time. Lucy, Bear and Beaver arrive and help them all down. They decided to let Little Crow be with the mother hawk, as they set off.
They soon arrive at a chasm, where they hear a voice, and help him out. The voice reveals to be Big Bear and he brings them to his home. Back at Granny's, Elizabeth and Jonathan made sure Granny's alive. It later reveals that Big Bear dug Granny's box out when he was digging. They thank him and leave for home, with Big Bear giving them his canoe as shortcut. They soon see Little Crow flying with Mother hawk, and the wise old turtle. They make it back just in time. They give her the item, her dad's fishing hook, a picture of her house she drew and a sealed picture of her as an infant with her parents. She smiles as she finally recovers. Later that evening, the family has picnic and they enjoy rest of evening together. The two turtles and Harriet dance to Granny's song, and reveal it's back to school and they'll both miss each other, hoping to see each other again real soon. As a reminder of their summer, Franklin gives Sam Granny's map and Samantha promises to never for get this summer as she kisses Franklin. The film ends picture of everyone, including Granny's, on the wall for all the goods that happened.
Franklin and the Turtle Lake Treasure is available in two versions, French and English, both of which are in different case covers.