Warning: Wikipedia contains spoilers
Vincent Dungarvan is a history teacher at a Catholic school for boys. Whereas his two brothers Matthew and Paul have been married with children for many years, Vincent is still single and living at home with his widowed mother, who is also a teacher. Although at 30 he is still a virgin, he is not asexual. He has gone out with one or two nice Catholic girls but has rejected them when he found them too superficial and boring.
One day, in the local library, he encounters Laura, a new librarian. Fascinated by her good looks and driven by, as he sees it, sinful desire, he is for once able to overcome his shyness and asks her out. Laura accepts, they immediately fall in love with each other and start dating on a regular basis. However, he prefers not to tell his possessive mother about her let alone invite her home.
In the course of the following weeks Vincent's life is shattered by a number of revelations concerning Laura. He is disappointed when she tells him that she is a Protestant and that, on top of that, she has stopped going to church altogether. What is more, through a deliberate indiscretion by Laura's flatmate Ruth, he learns that Laura is divorced. For him as a Catholic, this means that he is seeing a married woman, and both his guilt and his helplessness about the situation increase enormously. Accordingly, they break up their relationship.
Surprisingly, soon afterwards Vincent loses his virginity with Maureen, his sister-in-law, while his brother Matthew has gone out and the children are asleep. On the following morning, back at his mother's, he recollects what happened the previous evening:
In the end two instances of deus ex machina resolve the complicated situation. First, Maureen has a miscarriage, freeing Vincent of any doubt that he might have fathered an illegitimate child. Then Robert commits suicide, paving the way for a Catholic wedding between Vincent and Laura, who are planning to leave the past behind and start a new life somewhere else.