Chronologically, this is our introduction to Ensign Pavel Chekov. But because this one episode of the second season has such a low stardate, we don't see him again for several more episodes. This can make some sense if you believe his original posting was somewhere else on the ship and he was only brought to the bridge this one time before being assigned as the navigator at a later date.
A few things about this episode happen to support that theory. So many senior officers end up being off the ship that Kirk leaves a lower-ranked officer in charge of the bridge; Lieutenant DeSalle. DeSalle also regards Ensign Chekov as inexperienced at first. Perhaps DeSalle, who is an Engineer, has never worked with Chekov before, who is running Sciences during the bridge operation. And the whole time Chekov is using the Science station, someone else is manning Navigation. Because nothing is said or done in this episode that makes it seem like Chekov is part of the bridge crew yet, it works out.
spoilers for as-yet-unwatched entries
And this confirms he was already on the ship prior to the episode Space Seed, which removes a famous continuity error in Star Trek II: The Wrath of Khan.
