The most famous episode in franchise history, "The Trouble with Tribbles," is one of the highlights of the second season of Star Trek: The Original Series
. A deserved classic, the humourous story centers on an ever-expanding mass of furry creatures that memorably rain themselves down on top of Captain Kirk (William Shatner
) and into the middle of a Federation-Klingon showdown. It inspired one of the most memorable episodes in the spin-off series Deep Space Nine
, "Trial and Tribble-ations." Also in the second season, the Vulcan culture of Mr. Spock (Leonard Nimoy
) is fleshed out in "Amok Time" (in which Spock is faced with the possibility of killing his captain and friend) and "Journey to Babel" (introducing Spock's father, played by Mark Sarek, in what would turn out to be a long-recurring role). A new character, navigator Pavel Chekov (Walter Koenig
), was introduced; his Monkees haircut was intended to appeal to the younger audience, but he was also a Russian, which at the height of the cold war reflected Gene Roddenberry's optimistic vision of a more enlightened future. Other social-commentary opportunities presented themselves in "The Omega Glory," "The Doomsday Machine," and "Assignment: Earth," the last also one of those periodic opportunities to scrimp on the budget by time-traveling to an earlier version of Earth. Another example was "A Piece of the Action," a comic episode set in the Roaring Twenties and memorable for, among other things, Kirk's teaching a made-up card game called Fizzbin. In other significant episodes, "I, Mudd" saw the return of the bounder from season 1, "The Changeling" was the original inspiration for the first Trek feature film a decade later, "Wolf in the Fold" (penned by the author of Psycho) provides an example of the series' great writing, and "Mirror, Mirror" introduced the concept of the parallel universe inhabited by vicious, amoral counterparts of the regular crew, another theme later borrowed (more than once, and to good emotional effect) by DS9. --David Horiuchi
All 26 episodes from the second season of the Star Trek series that started it all. 'Amok Time' sees Mr Spock behaving most illogically: Kirk deduces that he must be returned to Vulcan for an ancient mating ritual. In 'Who Mourns for Adonais?', a giant hand grabs hold of the Enterprise, and its owner claims to be the Greek god Apollo. 'The Changeling' sees a space probe called NOMAD enter the Enterprise after mistaking Kirk for its creator. In 'Mirror, Mirror', Kirk and a landing party are transported into a savage parallel universe where Spock has a beard. In 'The Apple', Kirk and friends discover the people of Gamma Trianguli VI enslaved to a despotic computer shaped like a serpent's head. In 'The Doomsday Machine', the Enterprise crew rescue Captain Decker from the crippled USS Constellation. In 'Catspaw', Kirk and his crew encounter the mysterious but deadly aliens Sylvia and Korob. In 'I, Mudd', the galactic conman Harry Mudd is found living on a planet surrounded by beautiful androids. 'Metamorphosis' sees Kirk, Spock and an ailing Federation Commissioner forced to land on a planet where they meet Zefram Cochrane, inventor of the warp drive. In 'Journey to Babel', the Enterprise accompanies a number of diplomats to a conference, one of them being Spock's father, Sarek. In 'Friday's Child', the Enterprise crew race to prevent the Klingons forming an alliance with the inhabitants of Capella IV. 'The Deadly Years' sees the crew of the Enterprise subjected to rapid ageing through an alien disease, and a senile Kirk becomes unable to command the ship competently. In 'Obsession', a murderous cloud attacks the Enterprise, and Kirk realises he has encountered it before. In 'Wolf in the Fold', Scotty finds himself the chief suspect in a series of female murders. In 'The Trouble with Tribbles', Kirk and crew find themselves besieged with small furry animals while guarding a consignment of grain. 'The Gamesters of Triskelion' sees Kirk, Uhura and Chekov captured by powerful aliens, and forced to become gladiators for their entertainment. In 'A Piece of the Action', Kirk and company discover a planet where the culture is based on Earth's Chicago gang scene. In 'The Immunity Syndrome', the Enterprise discovers a huge space-born being that destroyed the USS Intrepid. In 'A Private Little War', Kirk discovers war on the planet Neural. In 'Return to Tomorrow', the crew of the Enterprise encounter some energy beings who want to borrow their bodies to help them construct androids. In 'Patterns of Force', Kirk finds that the Prime Directive has been broken, and a planet has developed in an unnatural way so that the society reflects that of Nazi Germany. In 'By Any Other Name', the ship is hijacked by a scouting party from Andromeda. In 'The Omega Glory', the Enterprise comes across the USS Exeter, a missing starship where the crew have been wiped out by a mysterious virus. In 'The Ultimate Computer', Dr Daystrom uses his new computer to play war games, but unfortunately the super-computer malfunctions - and begins to act on its own. 'Bread and Circuses' sees the discovery of a civilisation based on the Roman Empire on Earth. Finally, in 'Assignment: Earth', the Enterprise travels back to the 1960s, where it discovers an alien presence trying to alter time.