Into the Water World of Mongo

When their rocket crashes into the sea, an unconscious Flash is captured by water-dwelling people and surgically altered so as to breathe water rather than air. When Queen Undina discovers Dale's existence on a nearby island, she jealously orders her and Zarkov to be altered too. Once they help beat off an attack by Ming's forces, Flash and his friends are welcomed as heroes. When Flash frees Undina and Triton from the rebellious Pluton, she agrees to have their surgery reversed, and to let them go free.