Nurse Christine Chapel comes aboard the Enterprise on a journey to a far outpost to meet a man from her past that she has history with, who is now long dead and replaced by an android. After this encounter she decides to stay with the Enterprise working under Dr. McCoy and becomes a regular crew member.She had given up a career as a doctor of biology and taken a reduction in rank from Commander to Lieutenant in order to serve on the Enterprise.She eventually enters into a one-sided love interest with Mr. Spock, who is aware of her interest but, because of his Vulcan training (and unknown to everyone else, his childhood betrothal to a girl on Vulcan), cannot return her feelings.By the end of the original five-year mission, she has entered medical school and is well on her way to receiving her M.D. During the V'Ger incident, she is a full doctor and second only to McCoy among the Enterprise's medical personnel.Soon after the V'Ger incident, Christine Chapel transfers off the Enterprise to become the head of Starfleet Medical on Earth.