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.

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.

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 crew of the Enterprise races to find a way to stop a cloud from destroying inhabited planets.

The episode was rated 6.96 from 135 votes.

The Enterprise has as an observer on their current exploration/contact missions: Commander Ari bn Bem of the planet Pandro. He is rude and obnoxious and dangerously independent, but Kirk is under orders to treat him well to further relations with Pandro. After six months in his cabin Bem insists on going on a landing party. Trying to rescue Bem (who claims he is observing the primitive locals), Kirk and Spock are captured. Bem has replaced their communicators and phasers with dummies to observe them under stress as well. He reveals he is a colony creature who can split his body into separate, individually independent parts. Meanwhile, a sensory anomaly is sweeping over the planet, and appears to be a ""god"" of some sort that claims responsibility over its ""children"" and insists they leave. Bem escapes and Kirk rescues him again by telling the ""god"" that he has the same responsibility to Bem it does to her children. A chastened Bem acquiesces and the planet's natives are left to develop

The episode was rated 7.02 from 100 votes.

With Commodore Robert April (the first commander of the Enterprise) on board, Kirk pursues a ship plunging into a supernova, and enters a universe where time runs backwards.

The episode was rated 7.02 from 93 votes.

While exploring a burnt out supernova the Enterprise track a strange signal with the unrecognized word ""terratin"". They track it to the single planet orbiting the supernova and are hit with a burst of light from the volcanic planet. The crew and all organic matter begin to shrink while the ship's circuits are fused and the dilithium burned out. Struggling against a ship that they will soon be unable to control, they beam Kirk down to the signal source. The transporter recreates him from the original pattern, restoring him to normal size. The planet is plagued with volcanic activity and has a radiation that when focused causes organic shrinkage. Kirk finds a miniature city just before being beamed back up on automatic. He finds the ship deserted: the city below beamed the Enterprise crew, now at their same size down. The ""shrink attack"" was the only way they had to contact Enterprise, and they are about to be destroyed by volcanic activity. They were a lost colony: Terra 10 (i.e., ""terr

The episode was rated 7.05 from 109 votes.

Responding to a distress signal from a sector of space where ships disappear every 27.346 years, the male crewman are affected by a signal. Uhura and Chapel are forced to take command when the male crew onboard starting acting enraptured. On the planet's surface, Kirk, Spock, McCoy, and a redshirt are first greeted than captured by the women and given headsets that suck out their energies and cause their bodies to deteriorate as if they were aging. They escape but in their enfeebled condition are trapped in a giant urn filling with rainwater. Uhura leads a landing party that defeat the women readily and find the men. The women's physiology and life-energy requirements are keyed to the planet, and Enterprise agrees to take them to a normal planet. Using the transporter and the original patterns, they restore Kirk and the others.

The episode was rated 7.07 from 122 votes.

In orbit around a dead star, the Enterprise comes across a huge starship inhabited by a parasitic life form that threatens the entire ship.

The episode was rated 7.07 from 191 votes.

Kirk returns to the ""shore leave"" planet, but it's not as peaceful as it was when they last saw it.

The episode was rated 7.13 from 108 votes.

The Enterprise explores a mysterious cloud in space. After passing through it the ship's computer begins acting in a bizarre fashion. It replicates Kirk's shirts with ""Kirk is a Jerk"" on the back, manufacturers devices that leave a black eye ring on Spock, and shoot cream pies out of the food devices. Things take a turn for the worse when McCoy, Sulu, and Uhura are trapped in thea holodeck where they are beset by dangerous weather phenomena. They are rescued by Scotty when he bypasses the controls, but the Enterprise crew are further endangered when the Romulans attack and the ship's computer overrides manual controls. By feigning fear of the cloud, Kirk tricks the ship's computer into entering the cloud, which reverses the process. As they depart, they pick up transmissions from the pursuing Romulan ship that they are experiencing strange phenomena from their computer...

The episode was rated 7.19 from 99 votes.

The Enterprise crew go looking for the missing Ariel, a science ship. They track it to a class M planet. Kirk, Spock, and McCoy beam down and after passing through a series of oddly divided ecological regions, are captured by strange slug-like creatures. They are taken to a high-tech zoo where they meet the remaining three survivors of the Ariel. They are exhibits in the zoo belonging to the Lactran slug-aliens, who are highly intelligent. The creatures are so intelligent they don't recognize the humans and Spock as intelligent. One of the Ariel crewman is dying of an illness, and uniting their mental force with Spock they manage to contact a Lactran child to give them the medical kit and a communicator. The child snatches the communicator just as Scotty beams it up. While the adults concentrate their mental powers on Kirk in an effort to recover their child, almost driving him mad, the Lactran child takes over the Enterprise. After scanning its computers it beams back with Scotty and

The episode was rated 7.38 from 99 votes.

The Enterprise is escorting robot carriers loaded with five-lobed quinto-triticale to Sherman's Planet amidst rumors the Klingons haev a new weapon. The Klingons attack a small scout ship and the Enterprise rescue the single crewman: trader Cyrano Jones. And he has...tribbles. Neither Jones or the Klingons, led by Kirk's ""old friend"" Captain Koloth, will say why they want him dead. It turns out Jones escaped his sentence on Station K-7 (to clean up every tribble) by finding a tribble predator called a glommer. The Klingons attack the robot ships to stop the shipment and reveal their new weapon: an energy innervator that leaves both ships powerless. Meanwhile, McCoy checks out Jones' tribbles and finds out that the tribbles are safe...sort of. They don't reproduce. They just get bigger...and bigger. When Kirk beams the giant-sized tribbles aboard the Klingon ship, Koloth reveals a state secret: they created the glommer and want it back from Cyrano, who ""acquired"" it. Kirk obliges but at

The episode was rated 7.50 from 115 votes.

Dr. McCoy is arrested and held responsible for causing a plague that ravaged the planet Dramia Two, where he performed a mass-innoculation 19 years earlier.

The episode was rated 7.62 from 94 votes.

While on an away mission in the Guardian of Forever, Spock is erased from the timeline.

The episode was rated 8.02 from 162 votes.