Mirai shônen Konan (1978)

The story takes place in the 21st century, 20 years after World War 3. The world as we know it has been destroyed, and 12 year old Conan lives on an island with his grandfather, believing they are the only two survivors of the war. But one day Conan discovers an unconscious girl on the beach. She is 11 year old Lana, and she's fleeing from the evil rulers of Industria, who want to kidnap her and use her as a bait to convince her grandfather (who has fled from Industria himself) to give up the lost secret of solar energy, which would allow Industria to rule over the world. After Lana is kidnapped from the island, Conan decides to rescue her and sets sail to Industria.