The extended storyline which originated with "Space Seed" reaches a satisfying and enjoyable conclusion here.
As you might have noticed, this film is more upbeat that the previous two installments(perhaps a reason for it remaining the most financially successful of the Trek films?).
Three months have passed since our heroes brought Spock back and they were forced to stay put on Vulcan. The film opens with them deciding to face the music for their crimes in the last picture.
Like Back to the Future, the film becomes an enjoyable fest once the crew goes back in time and tries to save their present.
Catherine Hicks is delightful as the whale expert who gives them a hand, and the ending with Kirk being demoted to Captain and getting a new Enterprise bring this extended storyline full circle, and it is this dynamic-events of one film leading to another-which would make the Trek films stand apart from the series, and, in turn, would influence the directions taken by The Next Generation and Deep Space Nine(sadly, the films of TNG elected not to follow that example).