At Redwall Abbey, the inhabitants discover that the south wall is mysteriously sinking into the ground. Foremole Diggum and his crew believe the best thing to do is to knock the wall down and re-build it. During the night, a storm brings a tree down on the wall, making the moles' job easier but also leaving the Abbey open to attack. The broken wall reveals a well, which turns out to be part of the ancient castle Kotir. Abbess Tansy, Friar Butty, Shad the Gatekeeper, Giygas, and Craklyn the Recorder investigate below. After a harrowing journey, they find the treasure of Verdauga Greeneyes, the long-dead lord of Kotir.