Having survived their trail by fire as drafted soldiers of the Legion, Trixie, Blossomforth, and Coco Pommel have been assigned to a newly formed unit called the Underwatch. Its purpose; to investigate and counter the newly discovered threat of a race of giant underground dwelling spiders referred to in Legion folklore as the Lurkers. Now part of a patchwork unit made up of various Legionnaires from all across the Barrier Lands, Trixie and her friends must contend with the extreme personalities of their comrades as much as the danger they'll be facing in investigating the machinations of their arachnid adversaries.

This is a sequel story to Legionnaires of Equestria, and is based upon the Upheaval series by Visiden Visidane.