The story revolves around Nona Grey and the inhabitants of the Sweet Mercy convent, headed by Abbess Glass. Nona is a novice, along with her friends Ara, Juli, Darla and Zole, and in the previous book, the shipheart was stolen from the convent. The shipheart is a holy relic that actually powered the convent, and in this world where it is always winter and the ice is continually closing in on communities and causing neighboring lands to invade, it was a pretty important relic. The emperor's sister, Sherzal, was behind the theft and Nona lost a dear friend to murder during the chaos, so she is still healing from the events several years later, when this book takes place.
As Nona is studying to be a Grey Sister, that is, a nun well versed in subterfuge, she struggles more than the other novices. Perhaps it is because of her poor upbringing and the fact that she was sold to the convent, but regardless of her skill level, Abbess Glass is convinced the girl is better off at Sweet Mercy under their tutelage than off on her own. At least, everyone else is better off. Keep your friends close and your enemies closer? There's also Nona's little secret that she now carries an ancient demon named Keot inside of her, a very angry one who goads her into violence all too often. When the other members of the convent discover her secret, will she be kicked out?
Soon, it seems political machinations have worked such that the Sweet Mercy convent and Abbess Glass have come under suspicion of heresy, and Nona is forced out of the convent for breaking a serious trespassing rule. In truth, Nona and her friends have been exploring the caves under the convent where the shipheart was previously stolen and her friend killed, defying the mandate set out by Abbess Glass in order to find clues about the shipheart and finally get justice. However, a fellow novice and enemy of Nona has played her and the cost is Nona's place at Sweet Mercy. That or death, but banishment is a better alternative.
To complicate matters, Abbess Glass is arrested for teaching heresy, but due to Glass's vast bank of knowledge as a previous Inquisitor, she knows she can only be tried at a royal palace, due to some old rule of law. When she discovers she is being taken to Sherzal's palace instead of the king's, it would seem all is lost. But with Nona loose and on the run, and Sister Kettle and a few of the other novices working on Glass's behalf, expect nothing more than complete chaos at Sherzal's palace. Needless to say, things won't go exactly like Sherzal planned.
By the time Grey Sister comes to its explosive end, you are set up for the next book in the series, Holy Sister, almost as if the series should have been one huge book instead of three. While I enjoyed the story of Grey Sister, there is some serious world-building that must have gone on in Red Sister and since I didn't read it, I really missed out. For instance, not only do all of the nuns at Sweet Mercy have their previous names which they no longer go by, but are occasionally mentioned for certain characters, but they have their convent names such as Sister Kettle or Sister Apple. Then there are the titles bestowed to certain teachers, such as Mistress Shade or Mistress Path, but these characters also have their Sister names as well. I felt like I needed to make a chart when I first started reading the book, but eventually I figured they'd keep reinforcing those characters I'd really need to know about, so I didn't worry about it. There were also instances during Abbess Glass's trial and the trip leading up to it where previous interactions were mentioned offhandedly and again, they were lost to me since I didn't catch the references from the previous book.
All in all, if you have interest in this series, do start with Red Sister as I can't recommended Grey Sister as a starting point.
