I know Beverley Crusher came back in season 3, but Katherine was a big part of season 2 and she just abruptly left with no goodbye or anything.
The original ship's doctor had been Beverly Crusher, played by actress Gates McFadden. Apparently the writers didn't like Crusher's character and they had her written out at the end of the first season (but creator Gene Rodenberry left the door open for her return by having her reassigned rather than killed off). They bought in Dr Pulaski, played by Diana Muldaur, to be the new ships doctor. But the older and brasher Pulaski never seemed to catch on with fans, and McFadden was well liked by the cast, including star Patrick Stewart, so they eventually decided to bring back Crusher for season three.
