Claire is an English woman in her late twenties, on a vacation in Scotland with her husband after World War II. Both had been involved in the war and are reconnecting after years apart. During the trip, Claire stumbles upon a mystical stone circle that transports her to the same location in Scotland, only it is 1743. She immediately stumbles into a brewing fight between some English soldiers and some Scottish Highlanders. Claire gets involved and becomes a suspect to both sides, being English and wearing clothing that is far less modest than what was worn in the 1700s. The Scottish men take her for the time being, and she helps nurse one of them - Jamie - after he is injured in the fight with the English.
The book continues from there, detailing life in Scotland during that period and the numerous adventures that Jamie and Claire find themselves in. While some chain bookstores tend to shelve this in the Romance section, it is far from your typical bodice-ripper. Yes, there are some sex scenes, but they usually leave much to the imagination and tend to be more intellectual and emotional than graphic. Yes, there is the time-travel aspect, which causes some to categorize this with Sci-fi, but it is not a significant part of the book, with the exception of Claire's travels. If I had to label it, I would most likely classify it as historical fiction. Gabaldon conducts a great deal of research for every book she writes. Each book in the series typically takes her about three years to write, she says. Part of it is her writing style - she is very verbose. (At one point, I believe she planned this series to be five books, then seven, and recently she stated on her website that she is currently writing book seven and might have to extend it to eight!) But part of the three years is due to the extensive research she undertakes to ensure that what she is writing is true to the time period.
In simple terms, I adore these books.