Actually, the episode doesn't start off all that bad. An Enterprise landing party beams down to the newly discovered planet Phylos and discover several intriguing mysteries. They are soon confronted by the remains of the planet's population, a species of intelligent plants decimated by a disease brought to their planet by a human. The Phylosians save Sulu's life after he is poisoned by one of the planet's less intelligent species of plant life, then take the landing party to meet their "master."
This is when things start to get ridiculous. The landing party is attacked by shrieking purple dragon creatures which carry Spock away. Then the "master", a giant human who identifies himself as Dr. Stavos Keniclius 5, orders the landing party to return to the Enterprise. Reluctantly, they reply, but only to prepare to return to the planet and rescue Spock.
A search of historical records reveals that the "master" is a clone of Dr. Stavos Keniclius, a renegade scientist who escaped Earth in the wake of the Eugenics Wars. Since then, he and his giant clones have been searching for a "perfect specimen" to use in his plan to bring peace to the galaxy.
Now things get really silly. Returning to Phylos, the Kirk, McCoy and Sulu find Spock near death, with his mind nearly gone. Keniclius returns accompanied by a giant clone of Spock into which the original's mind has been transferred. Keniclius' plan is to create an army of giant Spock clones to impose peace on the galaxy. Fortunately, Kirk is able to convince the Spock clone that the galaxy has found peace in the time since Keniclius came to Phylos, and the clone uses a mind meld to return the original Spock's mind. The landing party goes back to the Enterprise, leaving the giant clones to work on the problem of rebuilding the Phylosians society.
If you think too much about this episode, which I wouldn't really advise, a couple of questions/problems present themselves. Why, for example, are the clones of Keniclius and Spock apparently over 100 feet tall? The episode never addresses this. Furthermore, how exactly does Keniclius plan to spread his giant Spock clones through the galaxy using the Phylosians space ships, which seem to be built for much smaller beings?
One good thing about the episode is the design of the plant-like Phylosians and their ancient city. These are prime examples of the advantages of the animated format in allowing the producers to create more exotic looking non-humanoid aliens than could be achieved in the original live action series.
Other than that, however, "The Infinite Vulcan" fails on pretty much every level. The episode starts with an utterly ridiculous premise, and executes it poorly.
On the other hand, if the real purpose of this episode was to make people look back on "Spock's Brain," and think that, in retrospect, maybe that episode wasn't so bad after all, then it succeeds admirably.