As one might expect of a film made in 1964 and starring Don Knotts, The Incredible Mr. Limpet is one of those nostalgic childhood favorites that absolutely fails to hold up. It's a twenty-minute premise that, at 99 minutes, feels like three hours. It's a simplistic plot, based on wimpy, nearsighted accountant Henry Limpet (Knotts) who has a fish-fetish. When he falls into the ocean at Coney Island, he is mysteriously transformed into a fish (complete with pince-nez). While Limpet the man is live-action, Limpet the fish (and the other underwater denizens he encounters) are animated, becoming an early entry into the vanguard of such mixed media (along with predecessors Fantasia and Song of the South, and in the same year that Mary Poppins used the same effects).
Mr. Limpet, in his fishy form, apparently has a bit of a super power, his "thrum" a sort of underwater bellow that can detect German submarines (this set during World War II), an ability he uses to aid Allied efforts to defeat Nazis in the Atlantic. Following this heroism, Limpet has the opportunity to return to his human life, and his overbearing girlfriend Bessie. His choice is made somewhat easier because Bessie has fallen in love with Limpet's best friend George (both understandably presuming Limpet dead), while Limpet has become enamored of the silky, coquettish Ladyfish (icky, I know). So the incredible Mr. Limpet stays a fish, and disappears for a long time. Later, needing his abilities again (this being 1964, likely against the Soviets), the US Navy seeks out Mr. Limpet and woos him back to serving his country.
As I've noted above, the movie is too long, too poorly acted, and too 1964. I remembered loving it as a kid, being entranced by the concept of turning into a fish, and by the then-highly unusual technique of mixed media. Don Knotts had the kind of aw-shucks febrile charisma that appealed to youngsters, and after all, Mr. Limpet was following textbook hero's journey - ignored and unimpressive character is transformed into something else, engages in valor and romance, and becomes legend. Mr. Limpet was essentially Captain America without the shield (or the arms and legs - but you get the point). I was disappointed that it's so dated and dull that my boys were bored and unimpressed, but I couldn't really blame them. Some things are best left to memory.