Space Seed is the first season episode of TOS which gave us Khan Noonien Singh, the eugenics wars back-story, and the situation that culminated in one of the better Star Trek films - The Wrath of Khan. For fans of the franchise, this is a landmark episode which established some important aspects of the history of the star trek universe. For non-fans, it is definitely one of the better TOS episodes, but perhaps not a legend or a classic.
The Enterprise encounters a 200 year old, pre-warp drive earth vessel derelict in space and boards it. Aboard, they find dozens of people in suspended animation. As the ship begins to power up, on of the stasis compartments begins to fail and Dr. McCoy saves the man within from the brink of death. The man is Khan Noonien Singh, a product of genetic engineering. This former world leader has been engineered to be smarter, stronger, and more aggressive than normal humans. What happens is somewhat predictable (except for the excellent finish), but very satisfying because of the Coon and Wilber script, the excellent performances, and solid Marc Daniels direction.
Khan is played with the pure intensity called-for in the role by Ricardo Montalban. Montalban carries every scene away, and Shatner and Nimoy - as powerful as their personalities usually are - do a great job of portraying respectful intimidation.
Highly recommended for franchise fans, recommended for others.