When Spock is stricken down by a disease fatal to Vulcans, the Enterprise is to meet a freighter with a load of drugs that provides the only cure, but Orion pirates attack the ship and Kirk must track down the Orions and the drug before it is too late.

The episode was rated 6.95 from 104 votes.

The Enterprise is investigating a ""Bermuda Triangle of Space"" when pursuit by three Klingon ships forces them into the area. They and the lead ship commanded by Captain Kor disappear and reappear in a mysterious null area of space filled with abandoned ships. They find a concentrated population on one ship and both captains are whisked away to meet with a Council of various aliens who are trapped there. Unable to escape because of the eventual drain of all higher power systems, they have formed a peaceful coalition, the Elysians, and insist the newcomers obey their rules. The Klingons fail in one attempt to escape the area, but Spock determines that the two ships together might succeed if they operate in tandem. Reluctantly Kor agrees but prepares to sabotage efforts. He has his mate plant a bomb during a social party between the two crews. The two ships linked together manage to escape but Scotty and Spock find the bomb and jettison it just before the Klingon ship breaks away and acti

The episode was rated 6.92 from 106 votes.

The Enterprise are sent to check out a conman who is their old friend Harry Mudd. He is trying to sell a fake love potion to miners. Kirk reveals the scheme and ""rescues"" Harry into protective custody. Mudd tricks Nurse Chapel into trying the love potion on Spock as a distraction to take her hostage and escape by shuttlecraft. Spock and Kirk pursue him to an abandoned planet but not before the potion is distributed into the air system. The potion has a delayed effect and Spock soon becomes enarmored of Christine. Complicating matters, a giant rock creature on the planet attacks and the Enterprise crew are too romantically distracted to rescue them. Kirk manages to use the love potion on the Godzilla-sized creature and it spares them long enough to be rescued. The potion's effects totally reverse and the love-smitten duos hate each other for an equal period of time. Realizing he had the real thing on his hands all along and didn't market it correctly, Harry confesses to his misdeeds.

The episode was rated 6.89 from 106 votes.

While carrying an ancient Slaver stasis box to a star base, Spock, Uhura, and Sulu get an indication that there is another nearby. Upon investigating, they are taken prisoner by the Kzinti - a cat-like race who hate humans after losing several wars to them. The Kzinti have an empty Slaver box, which is the only way to find another stasis box. They manage to open the Enterprise crew's box and find old meat, a strange hologram statue, and...a mysterious gun-like device. The item can shift its form. Spock and Sulu manage to escape but are recaptured when they trigger a matter-destruct sequence. An energy-draining setting almost lets Uhura escape. The Kzinti eventually stumble upon a setting that activates a sentient computer. When they are unable to provide the proper passwords it nonetheless provides them with another setting when they request the matter conversion form. But it's a self-destruct setting: the computer believes the Kzinti to be Slaver enemies and gives them the means to th

The episode was rated 6.81 from 100 votes.

The Enterprise investigate a seemingly-abandoned planet to discover it's inhabited by a race of plants, the Phylosians. They are then confronted by a giant humanoid, Dr. Keniclius 5, a refugee of the Eugenics Wars. He leads the Phylosians and they capture Spock and send Kirk packing. Keniclius plans to clone Spock as he cloned himself (being the ""fifth"" generation of the original) and bring peace to the galaxy via conquest. He's helping the Phylosians, a dying race, who are aiding him in return. McCoy whips up some weedkiller and the Enterprise landing party get past the Phylosians only to confront a giant Spock; Keniclius' first of many. But the original is to be sacrificed. Kirk goads the giant Spock into assisting the younger one by drawing on its reflexive actions. The clone-Spock revives the original with a mind meld and Keniclius gives up his dreams of conquest. He and the giant clone-Spock will stay behind to help the Phylosians.

The episode was rated 6.78 from 115 votes.

Near the Romulan border the Enterprise discover a one-man ship containing Carter Winston, a missing philanthropist. His fiancee Anne is aboard and the couple are happy to be reunited. But ""Winston"" transforms into an alien, knocks out Kirk, and takes his place long enough to order the ship into the neutral zone. The alien then takes McCoy's place to tell Anne to forget Winston and misdirect Kirk and Spock. They see through the ruse and realize ""Carter"" is a Vendorian, from a race of deceitful shapeshifters. It agreed to help the Romulans get hold of Enterprise. An outcast from its own race, it befriended the real Carter before he died and absorbed much of his feelings and emotions. It shapeshifts into a deflector shield to save Anne and the ship. At the end Anne suggests they try and make a go of it.

The episode was rated 6.75 from 120 votes.

While exploring the planet Argo, Kirk and Spock are turned into water breathers.

The episode was rated 6.62 from 98 votes.

Kirk leads a party to retrieve a religious relic that was stolen; however, he is faced with sabotage from within the group by someone who doesn't want to relic to be found, which will trigger an intergalactic jihad.

The episode was rated 6.56 from 99 votes.

The Enterprise is tracing the course of a probe that scanned Earth and sent a signal back along its course. They are attacked by a serpent-shaped energy field belonging to a feathered serpent who claims to be Kulkukan of Mayan-Aztec legend. It beams Kirk, McCoy, Scott, and Ensign Walking Bear aboard its ship to solve its puzzle before he will reveal himself. Aligning the mirrors in a strange city, they solve the puzzle and Kulkukan appears before them. It takes them to its ""zoo"" of violent creatures that now live in tranquility. Spock figures a way to break Kulkukan's force field and Kirk uses the diversion to free the animals. Kulkukan realizes that is not a god, but just a delusional old alien that once helped the younger races along. It leaves in sadness, leaving McCoy to remember the Shakespearean quote, ""How sharper than a serpent's tooth is an ungrateful child...""

The episode was rated 6.41 from 94 votes.

While on a mission at the center of the galaxy, the Enterprise are swept through the center of everything and end up in an alien world where all laws of nature and physics fail to work. They are rescued from total system collapse and death by a satyrlike creature named Lucien. He claims to be happy to have them finally come visiting, but preaches caution. Spock deduces that magic actually works in this universe, but the Enterprise crew's experimental use of it attracts the attention of Lucien's people. They came to Earth and practiced magic, but were hated, feared, and driven out. They put the Enterprise crew on trial in a mock Salem but it is actually Lucien they consider at fault. They agree to let the Enterprise go but condemn Lucien to eternal imprisonment. Defending Lucien, Kirk challenges the court ruling and uses magic himself to fend off the Chief Prosecutor. Impressed by Kirk's willingness to die for Lucien, who may have been the inspiration for Lucifer, the Megans send the En

The episode was rated 6.32 from 117 votes.