Grim Tuesday is the second book in the Keys to the Kingdom series by Garth Nix. It tells of Arthur's attempt to seize the Second Key from the eponymous Grim Tuesday. It is mostly set in the Far Reaches, the lowest part of The House and location of The Pit.
Arthur has returned home when the telephone that the first part of the Will (now known as Dame Primus) gave him starts ringing. Dame Primus informs him that in the six months of House Time that have passed since he left, Grim Tuesday, the second of the Morrow Days, has found a loophole in the agreement not to interfere with the other Trustees. This allows him to take control of the Lower House, which Arthur obtained from Mister Monday. Dame Primus tells Arthur that there is a way to overcome this loophole if he returns to the House, but the phone is cut off before she can tell him its nature.
Arthur then travels to the Far Reaches (Grim Tuesday's section of the House) with some difficulty, where he is mistaken for an indentured worker and forced to work. He then meets Japeth, a former Thesaurus. His work gang is forced to walk to another location, but Arthur and Japeth fall behind.
A vehicle arrives suddenly, containing Suzy Turquoise Blue. She tells them that she brought equipment to break into Grim Tuesday's Treasure Tower, so as to retrieve the second part of the Will and the Second Key. The three decide that Japeth should catch up with the work gang on Suzy's vehicle while Arthur and Suzy break into the tower. They reach the tower by crossing the ceiling of the Far Reaches, to find that the tower is surrounded by a giant glass pyramid. A large mass of Nothing which claims to be Grim Tuesday's former eyebrow, called Soot, gives them a diamond to cut through the glass pyramid, in exchange for helping it into the treasure tower. Arthur and Suzy break into the treasure tower, where they meet Tom Shelvocke the Mariner, the second son of the Architect and the Old One, who is currently Tuesday's servant as a result of blackmail. The Mariner, when requested, provides them with transport to a worldlet inside a bottle, in which the second part of the Will is located. They manage to retrieve the Will, which is in the form of a bear, and return to the treasure tower.
Grim Tuesday arrives and chases them through a weirdway (a type of distance-defying portal) into another part of the glass pyramid. They are then notified by one of Grim Tuesday's servants that the East Buttress of the Far Reaches is giving out, and that if not attended soon, it shall fall. Its fall will then lead to the destruction of all of them. Tuesday, whose power over the Far Reaches and his namesake day has been revoked by the Will, demands the Key to solve this problem; the Will, however, declares a contest between Arthur and Tuesday of creating something original with the Second Key, of which the Mariner is judge. The second key takes the form of two silver gautlets, which can be used to form objects and creatures out of Nothing. Whoever wins the contest could claim the Second Key and the Far Reaches. Tuesday creates a beautiful tree of precious metals; Arthur, knowing he can not compare in respect to physical beauty, creates a xylophone and plays a tune he composed. The Mariner, as judge, declares that while the tree is a great work, it was copied from a human sculptor; thus Arthur is the winner for having made something of his own.
Arthur goes to mend the eastern buttress, where he encounters a high-ranking Denizen, presumed to be Superior Saturday's Dusk. A fight ensues, wherein Arthur stabs his opponent, revealing that this figure, unlike most Denizens, has golden rather than blue blood. Arthur manages to mend the wall, stopping the buttress from collapsing. Once he returns, he is appointed Lord of the Far Reaches; as with the Lower House, he appoints Dame Primus (who now consists of parts 1 and 2 of the Will) his Steward and returns home. Dame Primus reverses the effects of the First Key on him before he left (to slow the process of him becoming a Denizen), at his request, and so he is in very ill health when he returns, and is sent to the hospital. When he wakes up, he finds an invitation from Drowned Wednesday under his pillow.