As best I can tell, Offcomer is a slightly delayed coming of age story for the more complicated matters of transitioning from late adolescence to adulthood on the psychological rather than the physical side.
The book portrays an indefinite period of Claire’s life, starting with her engaged in self-mutilation before heading off to her job as a food runner in a Belfast pub. Toward the end of the night a fight breaks out and one of the regulars, an old friend of the pubs owner and on good speaking terms with the staff, gets slightly injured. The owner calls a taxi for him and asks Claire to go home with him and make sure he is OK. He asks her to stay, they hook up, and then she heads home.
Through the remainder of the book we follow the outcome of that night, learning a bit about Claire’s past in the process. A bright student, a degree from Oxford, a promising relationship that sees her accompanying her boyfriend from Oxford to his new job as a professor in Ireland… and then working in a Belfast pub. She takes a break from the pub to go home for a bit and we discover remnants of a rural upbringing in the north of England, her friendship with the only other girl near her age in the village, and a strained relationship with her parents, heightened by her father having been afflicted with a condition in her childhood (perhaps a stroke, though it is not specifically mentioned) leaving him with significantly impaired mobility and unable to verbally communicate. Frustrated by not finding what she sought to by going home, she initially lashes out at precisely those things she had expected to heal her.
Then she is back in Belfast, picking up the pieces of her past existence and moving forward. It is not a fairy tale ending by any stretch, nor can you point to any particular passage as a significant turning point, but the Claire we encounter at the end of the book is very different than the Claire at the beginning. While rich in other descriptions, the book is surprisingly spare in describing that element – in many ways reflecting the real development of one’s mature self where the direct transformation is invisible.