A Librarian from the past teams up with the Librarians of the present to take down an ancient and powerful enemy let loose in the modern day. When superfan Flynn is met with the prospect of teaming with his Librarian hero Darrington Dare, he could not be more excited, until Darrington leaves the Librarians with this dire warning: There can be only one Librarian – ignore at your own peril.

When Baird goes after Nicole Noone in an attempt to repair her rift with the Library, she ends up joining Nicole in a mission to retrieve a dangerous artifact before it falls into the hands of Russian grave robbers. But can she convince Nicole to forgive the Library and return home with her? Or, will Nicole open Baird's eyes to the dangers of a Guardian dedicating herself, heart and soul, to the Library?

Lagertha is betrayed, and Bjorn must find a way to support the distraught queen; Floki and his settlers arrive in a new land, but it is far from what they expected.