The high-concept description would read simply: "Giant amoeba in space!", which pretty much sums it up. Hoping not to become the victims of an 11,000-mile-long single-celled organism as did the Vulcan starship Intrepid, the Enterprise crew attempts to figure out how to escape the organism's dark, mysterious void before being sucked into the protoplasm.
Dare I ask just where an 11,000-mile-long space amoeba came from? I don't think I will. Anyway, "The Immunity Syndrome" is well-executed starship-based entertainment. The core of the episode resides in Kirk being forced to choose between sending either Spock or Bones on a suicide shuttlecraft mission to study the organism from the inside, in the hopes of finding a way to destroy it. The engaging rivalry between Spock and McCoy, who both want to go on the mission, is particularly interesting ... it's an episode like this (in which, for example, Bones can't bring himself to wish Spock luck until after he has left the room), that makes the Spock/McCoy relationship so memorable.
As always, Sol Kaplan's score makes a big difference in selling the excitement, and Spock's sarcastic edge ("Thank you, Captain McCoy.") boosts the episode on the "pure enjoyment" scale. It's not particularly meaningful, but who said all Trek had to be meaningful?