We're Gonna Steal Mom's Diamond

While looking at her childhood photos, Mom comes across a photo of herself sitting on Grandma Kataoka's lap, who at that time was wearing a diamond ring. Mom tells Nobita and Doraemon that had the ring not been stolen by two thieves they would have been rich now, so Doraemon and Nobita travel back to July 10, 1948 (Shōwa 23) to protect the diamond.