Nobita and his friends under the waters of the Pacific Ocean. Gian and Suneo take Doraemon's underwater vehicle and travel through the Atlantic Ocean, trying to find a treasure ship. Along the way ...

Because the monkey king is only a legend, Nobita and Doraemon have decided to go back in time using the time machine and make a fake one and show Gian, Shizuka, and Suneo to prove that they are right.