Kelly Sullivan comes to Ireland from America to facilitate the transfer of ownership of Whitlock Castle to the hotel company for whom she works.  The family currently in possession of it is none too pleased with the renovation plans she brings with her for their heirloom, but they really have no choice but to sell.  After initially taking an instant dislike to one another, Kelly gradually begins to win over and be won over by the O'Mearas, especially the son, Connor, who is in charge while his father is in the hospital. Once she has fallen in love with the castle and in at least lust with Connor, she vows to try to preserve Whitlock Castle for his family even after her boss takes control of it.

Breaking company rules to have a personal relationship with Connor breaks through Kelly's own code of conduct. She begins to do manual labor and stops eating like a bird. She begins enjoying life's small pleasures and finds the corporate world holds less and less appeal. Yet despite loving Connor and his whole family, his father, his two aunts, and even his old girlfriend, she feels she must return to America and see to her job.

A promotion for her success in Ireland awaits her, but she does not find the pleasure in it she had hoped to. Her joy lessens further after Connor and his father follow her home.  After several weeks, she finds an excuse to return to Ireland, even though it means brushing off an "obligatory" company social function and the advances of the president. At a town festival, she comes face to face with her boss. He is displeased with her, but promises to consider not modernizing the castle and selling off family heirlooms if she will return immediately to the United States and break off with Connor. Reluctantly, she agrees.

However, the boss goes back on his word, and when another executive is given the Ireland job and tells Connor's father of the modernization plans, the older man suffers another stroke. Connor pulls out of the sale, and Kelly quits her job to return to the man she loves.  Though the happily ever after is a bit more on the hardship side than most, it does all work out in the end.

Midsummer Lightening has some of the appeal of "Witness", with its clashing cultures creating the conflict between the hero and heroine, but without the suspense. Kelly was hard to like, as she tried to cling to her more shallow values, but the Irish cast was readily appealing. Small towns are not much different across the sea as ones in America. Kelly has to deal with everyone knowing her business, down the color of her underwear, and dealing with the whole town's opinion of her, but manages it well. I would have liked it better if it had some kind of suspense or a more likable heroine, but it was a fairly good story.