I love me a good time-slip novel, and Susanna Kearsley writes some of the best. This one involves an archeological dig of a site that may have belonged to the lost IX legion. Since no one has any idea where the IX Legion served, southern Scotland is as likely as anywhere. This particular site is a private dig, paid for by a wealthy man. His grandson sees and hears things that may be Latin, and an individual he calls the Sentinel.
This is the framework for this romance between a female archeologist and a local man, and seeing how they work it all out is part of the fun of a Kearsley novel.