A sorceress uses what’s called “sympathetic magic” to hold an amulet of the Enterprise over a flame. Inside the real Enterprise, the crew starts to sweat as the temperature inside the ship rises critically high. And that’s just one scene from the episode which is considered Halloween-esque. (It even aired on October 27, 1967.) Add to this an oversized cat, mind control, and aliens who end up looking like decaying worms, and you have the essence of Catspaw.
Sylvia is an alien who uses a transmuter to allow her to control matter, but she isn’t happy with her previous form as she can’t feel sensations. So she uses her magic to change herself in hopes of enticing Kirk. Of course, Kirk doesn’t fall for it, and instead, uses his charms to gain information, and with the help of Sylvia’s partner, Korob, manages to free the Enterprise and escape.
In spite of the absurdity, Catspaw teaches a very important lesson. We can’t take from others to make our lives better. Sylvia was so unhappy with her life, and she thought, that by using her magic, she could create the perfect life. But that isn’t how life works. There will always be thieves who think stealing from others will enhance their own lives, but ultimately, that’s not where true happiness lies.