"Help... can't... breathe!"
On a mission, Kirk's and Spock's DNA has been altered to make them only able to breathe underwater. So, they explore the underwater world in order to undo the damage done to them.
This episode provides examples of:
- Anatomically Ignorant Healing: Kirk and Spock are captured by a giant marine monster, and a second team is dispatched to rescue them. When the two senior officers are found, both have been fitted with gills and webbing between their fingers, and can no longer survive out of the water. The Aquans that inhabit Argo are xenophobic, especially of air-breathers, but took pity on Kirk and Spock, restructuring them to survive in a marine environment. Much of the episode revolves around efforts to undo this process.
- Chekhov's Gunman: Early in the episode, the aquashuttle is attacked by a sur-snake. Kirk and Spock encounter the creature again during the climax.
- Cruel and Unusual Death: After the Aquans capture Kirk and Spock, and after they come to the conclusion that they are there to destroy them like previous outworlders, chain them down with a net on dry land. Since they've been modified to breathe water at that point, they were slowly suffocating to death until they got help from Scotty.
- Fish People: The Aquans.
- Mermanity Ensues: Kirk and Spock are turned into Fish People by the Aquans when the two are captured by a giant sea monster, but they understandable want to go back to normal (Kirk can't command a starship from an aquarium!). They discover the Aquans did this to themselves many years ago to survive the planet's environment.
- Mobile Fishbowl: At the end of the episode, two Aquans are shown on the bridge of the Enterprise wearing water-filled helmets on their heads.
- Screw the Rules, I'm Doing What's Right!: Some of the Aquans break the "ordainments" to help Kirk and his crew capture a sur-snake so that its venom can restore Kirk and Spock to normal.
