I honestly don't know why I keep reading this series, but I do.
The general premise of the series is: special ops in kilts. It is set in the time of Robert the Bruce, and the Highland Guard is an elite group of shadow warriors who are basically invincible. They do not contain any paranormal stuff - just tall, muscular highland warriors who kill enemies. The heroines are slightly more interesting than the heroes, who tend to be pretty interchangeable: emotionally unavailable warriors who aren't interested in falling love because weakness.
There is quite a bit of historical content to the novels. I have heard varying reports on their accuracy, but there is definitely a modern bent to the dialogue and to the attitudes of the heroines. I don't read historical romance for anything other than total escapism, so this doesn't really bother me. But it's a warning for those who care - Monica McCarty is not Sharon Kay Penman. She does include notes at the end of the books where she discusses her historical sources, and where she has fictionalized things, which I find interesting and enjoyable.
So, The Chief. This is the first of the books, and tells the love story of Tor McLeod, the leader of the Highland Guard, and his wife, Christina. Christina is the daughter of a fairly terrible man who is abusive and forces her to trick Tor into marrying her. Before she does this, she gets him to agree that if she is unhappy with the marriage, he will allow her to enter a nunnery.
I'm going to digress for a moment to talk about sex. I don't know if we have reached peak sex in romance novels or not, but I have grown increasingly weary of the obligatory torrid sex scenes contained in all romance novels. Monica McCarty is guilty of some of the most rote, repetitive sex scenes in romance. Her scenes invariably contain similar words and literary imagery. This didn't really bug me until I hit the fourth book, Viper, when it just became so apparent that she was recycling. But I can't really blame her, because how many different ways can one author describe the same physical act of sexual intercourse? The problem, in my opinion, isn't necessarily Monica McCarty. The problem is the unspoken default requirement that romance novels include explicit sexual content, which is so often boring and repetitive.
Of the four that I have read, The Chief is probably my favorite. I really liked Christina. In spite of all of the complaints that I have articulated, I enjoy these books. This second time around, it is actually the overall narrative arc of the books that I am enjoying the most - the story of Bruce's bid for Scottish independence, the Highland Guard and their battles and intrigues and set-backs. I have also enjoyed the character development of the female characters, who are often given substantive roles in the rebellion. I have also enjoyed a relative lack of shaming of other women by the heroines - there is a valuing of female friendship that can be lacking in romance novels, where the novelists tend to pit the heroines against all other women as challengers for the heart of the hero.