"The World Beneath the Waves"
The Enterprise is ordered to rendezvous at "space sector J-17, R-41" in order to take on a new technical advisor, and a new assignment. The crew is less than thrilled. "Haven't we got enough sci-techs of our own without them sending us some white whiskered authority on cross-eyed butterflies?" asks Scotty. The advisor turns out to be Barbara McCoy, Leonard McCoy's daughter! (Last seen in issue #40) She tells Kirk that the Enterprise has been assigned to study the planet Bwuja.
Later, McCoy and his daughter talk in her quarters. The Doctor is eager to get to know her better, but Barbara is bitter that her childhood had been so difficult. She blames her father for divorcing her mother. The two get into a bitter argument. Kirk breaks up the fight, saying "Life on a starship is confining enough without that heat! You'll handle this better, or I'll remove one of you!". The two agree to behave.
When the disaster came, only a small portion of the population was able to escape in time. The devastation on the surface was total. Even the oceans boiled and steamed, but the Bwujans lived in great natural caves in the deepest trenches of the ocean. In time, the surface cooled, but no one sent to the surface returned, or did so the victims of extreme radiation. The Bwujans lived that way as slaves of the gill transplant for 1,500 years, until finally the children began being born with natural gills! There were rumors that these water-breathers even killed off the original population. Though the surface is now free of radiation, the Bwujans still fear it. "Our assignment is to study this amazing people who have experienced the fastest complete physio-adaptation known!", says the tape.
The Captain, Dr. McCoy, Barbara McCoy, and Mr. Spock beam down to the planet's surface, amidst the ruins of the cosmic catastrophe. The Doctor gives them all "Comp-Ox" (compressed oxygen) capsules which enable them to remain underwater, for 24 hours and they enter the ocean and swim into the depths. At 2,600 feet down (wearing no protection other than goggles and wet suits!), they are met by three guards riding large sea creatures. The landing party is led to a huge underwater city, and taken before King Raan XIV and his queen, Saya. The King demands to know why they have come. Barbara tells him that they wish to learn from him, and offer their friendship. King Raan distrusts them, and says that "It was the air breathers of our own world who made the gods angry and destroyed our surface land!". He orders the landing party be locked away to await execution in two days.
The crew is locked in a large stone cubicle, and a pump removes the ocean water from the cell. Spock hears tapping on one of the walls. They quickly use their phasers to cut a hole in the wall. They discover a group of Bwujans on the other side breathing air! The Bwujans explain that they are "remutations", or abnormal births, among the water breathers. The priests of their society have decreed that they must die at birth, but the King has secretly been banishing them to the "air cavern" instead, to live out their lives. The air breathers tell Kirk that many of their people have tried to escape by climbing up a rocky shaft that leads all the way to the surface. Most of these "climbers" turned back when they reached the cave of Agaaras, a vicious atomic mutant created by the catastrophe.
Kirk decides to try and escape from the cave by climbing the shaft, and the landing party begins the long ascent. They arrive at a great rock chamber, and are immediately attacked by the monster Agaaras! (Who looks like a large, blue-green armadillo) Kirk sets his phaser on "Max-D" and fires at the monster, but it is unaffected. The beast approaches Barbara, and McCoy distracts it so that she can escape. Just as Agaaras is about to attack the Doctor, it falls apart into hundreds of pieces on the ground! Kirk explains that the phaser shot must have killed it, but the beast was held together briefly by some fantastic cellular bond. The group continues upward, and soon reach the surface of the planet.
There, they find a young air-breathing Bwujan boy. "He was no climber, Captain!", says Barbara "Too young for that!". "Then how did he get here?", asks Kirk, "unless he was - born here!". The boy explains that his mother and father were both climbers, and leads the group to his tribe in a great cave. They are taken before "The Mighty Doro", chief of the air breathers. (Who is a child of three or four) Kirk tells the boy's father Tako of their adventure, and asks him why he has not sent an expedition back down the tunnel to bring up the rest of his people. Tako explains that if the King learned of their existence, he would send troops to the surface to kill them all. At that moment, Spock yells for help from outside the cave.
Kirk rushes outside, and finds Spock being held at knife point by King Raan from the city below. (Who apparently has no trouble breathing air.) Raan tells Kirk that his priests had warned him that someday the air breathers might reach the surface of the planet. "I mean you no harm, but the King of the air people must die!", he says. Raan is brought to Doro, and discovers that he is only a boy.