The Enterprise finds the colonists of Omicron Ceti III in perfect health, a symbiotic spore protecting them from the effects of Bethold radiation. The spores also, however, produce some other unusual psychological effects on their hosts.
Mr Spock's fans wanted him to show some emotion, they got their wish in this episode.
The first draft script was called 'The Way of the Spores', and it was Sulu who fell in love instead.
Leila Kalomi is played by actress Jill Ireland.
Kirk's speech on the destiny of mankind qualifies him for the BAFTA award for 'Most clichés strung together in a single speech by a starship captain'. He uses five in a row: 'fight our way through', 'claw our way up', 'scratch every inch of the way', 'stroll to the music of the lute', and 'march to the beat of the drums'.