In the 23rd century, inhabitants of a domed city freely experience all of life's pleasures — but no one is allowed to live past 30. Citizens can try for a chance at being "renewed" in a civic ceremony on their 30th birthday. Escape is the only other option.
When a destructive space entity is spotted approaching Earth, Admiral Kirk resumes command of the Starship Enterprise in order to intercept, examine, and hopefully stop it.