Something’s gone fishy! Spock and Kirk have gills! The Enterprise’s cool aquacraft is attacked by a flaming squid monster and some Mer-pacifists save Spock & Kirk (against the protestations of their uptight reactionary leadership who don’t trust those nasty air-breathers!) Then there’s some race against the clock and everyone is happy at the end. I kinda forget. But I love seeing Kirk considering to command the Enterprise from inside a fish tank!