As helmsman, Sulu pilots the U.S.S. Enterprise during its five-year mission. As third officer, Sulu often takes command in the absence of superior officers.
Arguably Starfleet's best commanding officer, Captain Kirk is the sense of authority in the Enterprise's initial five-year mission to boldly go where no man has gone before.
As Chief Engineer, the no-nonsense Scotty works miracles in repairing and maintaining The Enterprise. Scotty also takes command in the absence of Kirk or Spock.