Plot SummaryEditSPOILER ALERT: Plot Revealed
Taken from Wikipedia 
The main novel begins at Abhorsen's House, which is besieged by Dead Hands led by Chlorr of the Mask, once a powerful necromancer who has died and come back as one of the Greater Dead. She is in turn under the control of Hedge the Necromancer, who serves Orannis the Destroyer. The Destroyer is the Ninth Bright Shiner, and the most evil magical force or being. It had destroyed many worlds before It was defeated by the Seven Bright Shiners, the free magic entities that formed the Charter after defeating Orannis. The Seven also bound Yrael, the Eighth Bright Shiner, who was a free magic entity who would not join the charter. The Seven were known as Ranna, Mosrael, Kibeth, Dyrim, Belgaer, Saraneth, and Astarael. These Seven binders are also the names of the seven necromantic bells and some of the original natures of The Seven linger in these bells. Lirael and Sameth must escape the Abhorsen's House to stop the Destroyer and to save Sameth's friend Nicholas Sayre, who is being used by Orannis as an avatar.
Lirael and Sameth now have to travel to the Red Lake, where The Destroyer is being unearthed. As Lirael and Sameth journey through the Old Kingdom they learn more about Orannis and its plans to destroy all life.
Meanwhile, Prince Sameth's parents, the Abhorsen Sabriel and King Touchstone are in Ancelstierre trying to stop the probable death of thousands of Southerling refugees if they are allowed to enter the Old Kingdom without the protection of the Charter (see also the Five Great Charters). While they are in Ancelstierre attempting to reason with a corrupt government they become victims of an assassination attempt and barely escape with their lives. They flee to the Old Kingdom to attempt to save the lives of the Southerling Refugees from the other side of the Wall.
While Sabriel and Touchstone are trying to get back to the Old Kingdom, Lirael, Sameth and the Disreputable Dog are trying to save Nicholas Sayre, Sameth's best friend and also the host of the Destroyer. The question becomes one of whether Lirael and Sameth are able to stop The Destroyer from completing its plans for eternal freedom and the destruction of the world and other worlds after this. Orannis is successful in joining the hemispheres that imprisoned him. Nick dies in the process of the rejoining, but the Disreputable Dog gives him a Charter Mark, thus binding him to the edge of death. Lirael uses her Rembrancing powers to figure out how the original Seven bound Orannis. During her journey through Death to use the Dark Mirror, she is confronted by and defeats Hedge. In the end, Lirael and her friends defeat Orannis, who must once again bind Orannis by re-enacting the original binding of the Seven with each member holding a bell and adding a bell’s voice. Lirael takes Astarael, and prepares to strike at the hemisphere with a new sword, forged from her panpipes and Nehima, that Sameth made for her. The others include King Touchstone (Ranna) and Abhorsen Sabriel (Saraneth), Sanar and Ryelle (Mosrael), Ellimere (Dyrim), the Disreputable Dog (who is truly a remnant of Kibeth), and Prince Sameth (Belgaer). The first attempt at rebinding shows that the Destroyer is strong enough to resist. Eventually, Sameth frees Mogget, who reveals himself to be the Eighth Bright Shiner, Yrael. Yrael fights against the impulse to kill the Abhorsen, and sets itself against Orannis. This shocks Orannis, and gives the added power to bind him. As Lirael prepares to make the final blow, she readies herself to die. Unexpectedly, the Disreputable Dog takes the blow for Lirael and disappears into Death. In the end, though, the Disreputable Dog gives Nick back his life and tells him that Lirael will have a hand of gold made for her by Sameth to replace the hand she lost while being saved by the Dog.