The Mule searches for the Second Foundation — a secret group of mental scientists hidden somewhere in the galaxy. The First Foundation also hunts for it, afraid of being controlled. Asimov layers deception upon deception in a chess match between psychohistorians, mutants, and ordinary people. The original trilogy's brilliant conclusion.
Clean.
Skip if you dislike:
Moods: Cerebral Twisty Deceptive Satisfying
Tropes: Hidden Organization Mental Powers Double Bluff Where Is It?
Yes — the first three Foundation books form a complete arc.
The answer is one of Asimov's most celebrated reveals.
The original trilogy is self-contained — later books connect to Asimov's other series.