In Mortal Coil, Skulduggery and Valkyrie discover that the Arbiter, a powerful being who helps maintain the balance between good and evil, has disappeared. This event threatens to tip the world into chaos, as various factions begin to vie for power.
Skulduggery and Valkyrie race to find the Arbiter and restore the balance of power. Along the way, they must face new and old enemies, including the deadly sorcerer Serpine and the mysterious Mr. Bliss.