Intent on returning home to Earth to face trial for their actions in the previous film, the former crew of the USS Enterprise finds the planet in grave danger from an alien probe attempting to contact now-extinct humpback whales. The crew travel to Earth's past to find whales who can answer the probe's call. It is the fourth feature installment in the Star Trek film series, and is a sequel to Star Trek III: The Search for Spock (1984); it completes the story arc begun in Star Trek II: The Wrath of Khan (1982) and continued in The Search for Spock.

References:
https://en.wikipedia.org/wiki/Star_Trek_IV:_The_Voyage_Home