In a small coastal town, Henry Limpet is a rather peculiar guy: an ordinary man who longs to have adventures in the ocean, although his job at a fishing supply store doesn't offer him many opportunities. After being rejected by the army due to his clumsy demeanor, Henry feels useless and frustrated. One day, while walking along the beach, he is pushed into the water and, in a magical twist, transforms into a charming and tiny sea creature, a fish.
Now underwater, Henry not only discovers his true nature, but he also becomes a great ally to the United States Navy during wartime. With his new body, he can do what he has always yearned for: explore the ocean, meet other fish, and even fall in love with a beautiful fish, leading to many comedic and heartwarming situations. But his underwater life is not just fun, as enemy submarines threaten to endanger the security of his country.
Taking advantage of his new form, Henry gets involved in secret missions, using his cunning and skills as a fish to thwart the enemy's plans. Amid laughter, fun, and a bit of aquatic romance, this animated action-adventure turns into an inspiring story about courage, friendship, and the search for purpose in life, uniting humans and sea creatures in a struggle for the greater good.