Which Star Trek film was Leonard Nimoy's feature-film directorial debut?

Star Trek III: The Search for Spock was Leonard Nimoy's feature-film directorial debut. Released in 1984, it marked the first time a Star Trek cast member directed one of the franchise's theatrical films.

Nimoy had already played Spock in the original television series and the first two films, but his directing experience began earlier in television. His first credited directing work was the “Death on a Barge” segment of the 1973 Night Gallery episode “The Different Ones.” Star Trek III was therefore his first feature-film directing assignment, not his first directing job overall.

The film continues directly from Star Trek II: The Wrath of Khan. After Spock's death, Kirk learns that Spock's katra, or living spirit, has been transferred to Dr. Leonard McCoy. The Enterprise crew steal the decommissioned ship and return to the Genesis Planet in an attempt to reunite Spock's body and spirit.

Nimoy's success led Paramount to hire him again for Star Trek IV: The Voyage Home, which he also directed and co-wrote. The distinction between Star Trek III and IV often causes confusion, but III was the debut; IV was his second feature and became an even larger commercial success.