What was the name of the episode where the Enterprise encounters a giant space amoeba?

The episode where the Enterprise encounters a giant space amoeba is “The Immunity Syndrome.” In the story, the Enterprise investigates the destruction of the USS Intrepid and discovers an enormous, energy-draining organism drifting through space.

The creature initially appears to be a gigantic multicolored object, but a sensor probe reveals that it is alive and made of protoplasm. It absorbs energy from ships and planets, creating a threat that could spread across the galaxy. Spock ultimately pilots a shuttle into the organism to study its interior and identify a way to destroy it.

The title refers to the organism’s ability to resist attacks, much like a biological immune system. The episode is sometimes confused with other giant-threat stories from the original series: “The Doomsday Machine” features a planet-killing machine, “The Changeling” centers on an advanced probe, and “The Corbomite Maneuver” involves a deceptive alien encounter. “The Immunity Syndrome” first aired during season two.