Abeth’s dying red giant sun cannot generate sufficient heat to prevent a global ice age.

Abeth’s man-made moon refracts sunlight onto a narrow strip of land circling the globe.

At Sweet Mercy, Nona trains with other novices including Arabella "Are" Jotsis, a noble, and Zole, a girl from the ice tribes.

Nona learns that Zole's bodyguard Yisht is attempting to steal a valuable artifact from Sweet Mercy: the shipheart, which was left by the original settlers of Abeth.

In a frame story, an adult Nona and Ara are attacked by Noi-Guin assassins and Lano Tacsis, Raymel's brother.

Nona, Ara, and their friends explore the caves under the convent in defiance of Abbess Glass’s orders.

Nona discovers a portal that can take her back to Sweet Mercy; Zole remains on the ice.

Nona reveals that the entire scheme of stealing the book was a ruse to force Sherzal to bring the Sweet Mercy shipheart to the palace.

Nona focuses the moon to incinerate the Scithrowl queen; the remaining combatants flee the city.

Writing for Den of Geek, Bridget LaMonica praises Lawrence's subversion of standard fantasy novel tropes.

Lawrence believes that reading the Malory Towers series by Enid Blyton with his daughter inspired him to focus on an all-female school setting.

[3] Writing for Tor.com, Laura M. Hughes declared that Red Sister was an improvement from Lawrence's previous works.

[5] Writing for Library Journal, Kristi Chadwick wrote that Red Sister contained a "fantastic world in which religion and politics are dark and sharp as swords".

[6] In her review for the Sydney Mechanics' School of Arts, Melanie Ryan categorizes the novel as grimdark and praises the "exquisitely crafted" world, plot, and characters.

[7] Publishers Weekly wrote that Grey Sister "will keep readers hooked" and that Lawrence's use of "small insights" into his characters lives "elevates the second volume ... above similar books featuring a young woman who is separated from her family".

[8] Library Journal gave the second novel a starred review, stating that "vivid worldbuilding and fast-paced action enhance this powerful coming-of-age story".

[10] Bridget LaMonica praised Holy Sister's exploration of the themes of corruption and power, as well as the moral ambiguity in the story.

[11] Publishers Weekly felt that Holy Sister was a "pallid conclusion" to the trilogy, with "uneven prose and stock characters" that "[fail] to live up to the series’ promise".