Once every thirty-three years, a rift in space opens a narrow window to the Calligar, a reclusive species the Federation has never managed a lasting relationship with, and Kirk's crew has this one brief pass to get it right. When Spock's delegation is taken hostage and the Calligar reveal a secret with teeth, Kirk has to defuse a standoff on both sides of the rift before it closes for another generation.
Summary by Loreful, from public sources