The Enterprise's senior physician during the voyage to the galactic barrier, serving before Leonard McCoy joined the crew.
Mark Piper is the chief medical officer aboard the USS Enterprise when the ship attempts to cross the barrier at the edge of the galaxy. After the crossing leaves Gary Mitchell and Elizabeth Dehner with growing psychic powers, Piper treats them and monitors Mitchell's alarming changes.
He accompanies the party that takes Mitchell to the automated station on Delta Vega, and is caught up in the chaos as Mitchell escapes. By the time of the following episodes, Leonard McCoy has taken over as the ship's doctor. Piper was played by Paul Fix.