The mission of Mendoza and Joseph (plus a third operative) is to travel to medevial England, posing as Spanish nobility. They are to stay with Sir Walter, who owns a garden of rare plants, which he calls the Garden of Iden, and which he charges a couple of pence to take a tour. Unbeknowst to Sir Walter, many of his plants are rare, but have valuable medicinal properties, and Mendoza wishes to harvest them and send them forward in time for the use in the 23rd Century. In the household of Sir Walter is Nicholas Harpole, who is a Protestant. He is supposed to be reformed, and now a practicing Catholic (it is the middle of the 16th century, and Mary is on the throne of England and has decreed that the official religion is Catholic. Heretics are burned, thus it is wise for Nicholas to hide his Protestant background.)
At this point, the novel slows down. For some reason, it takes Mendoza seasons to do her plant cuttings. What is so hard about snipping a few leafs? Why not just dig up a whole plant and hightail for the border? But Mendoza and the novel dwaddle around Sir Walter's household, allowing the 19 year old Mendoza to inevitably fall in love with the mortal Nicholas. Even though Mendoza is now immortal cyborg, emotionally she is still a young adult, and this is her first romance. And so we get some long chapters about her budding romance with Nicholas. Nicholas suspects somethings are quite right. Sometimes the members of the Company inexplicably lack information that should be common knowledge, or they behave in ways inappropriate for citizens of that era. Other reviewers on the Amazon website find this material wildly hilarious. I guess it depends on your cup of tea.
Of course, things end badly. Mendoza is immortal and Nicholas is not. England is a violent and dangerous place. And so we zip to the tragic end. This seems like a lightweight novel - you will zoom right through, like it is just a long novella. I suppose it would be called a "character" driven story, since it isn't much of a plot once Mendoza becomes a cyborg. If you want to read a really rocking time travel story, try The Anubis Gates by Tim Powers. That book is terrific!