Grandpa 2.0: Invincible at Ten

In 1976, Mac Sullivan takes his wife and daughter to the countryside. While he is out gathering wild vegetables he comes across two sages at a chessboard. He plays a single game in one of their places. When it ends, 49 years have passed in the world he came from. One of the sages gives him a peach of immortality to make up for the time. Its power is too strong for him and it turns him back into a ten-year-old, with decades of chess in a child's body. He is a nine-time world chess champion. He goes home in the boy's body and the bodyguards stop him at the door. His grandson bullies him and his son-in-law talks down to him. To prove who he is, he solves a chess position, and he uses the peach to wake his wife. Behind the family's troubles there is something else being kept from him.