Clockwork Princess

The final installment of The Infernal Devices trilogy, set in Victorian London's hidden world of Shadowhunters, half-angel warriors sworn to protect humanity from demons, resolves the intertwined fates of Tessa Gray, a shape-shifter of mysterious origin; Will Herondale, a tormented Shadowhunter; and Jem Carstairs, Will's gravely ill parabatai, a warrior bound to him by sacred oath. Their enemy, Axel Mortmain, a mundane (nonmagical human) inventor bent on destroying the Shadowhunters with an army of clockwork automatons, tightens his grip as the story opens.

Tessa is being fitted for her gold wedding dress at the London Institute, the Shadowhunter headquarters, when Gabriel Lightwood, a young Shadowhunter, arrives covered in blood. His father, Benedict, has transformed into a giant worm, the final stage of demon pox. The Institute's fighters ride to the Lightwood estate. Will's younger sister Cecily, who recently arrived from Wales hoping to bring Will home, leaps onto the worm's back and drives a dagger into it before Gabriel fires the killing arrow. Afterward, Jem collapses, coughing blood.

The Silent Brothers, the Shadowhunters' order of healers and priests, stabilize Jem, but the situation is dire. Jem confesses to Will that he has been consuming two to three times his normal dose of yin fen, a demon drug that sustains his life but cannot cure him, desperate to feel fully alive for Tessa. A year's supply is gone in eight weeks. When Will searches for more, he discovers Mortmain has bought up the entire supply.

Mortmain sends a letter to Charlotte Branwell, the Institute's head, offering to trade yin fen for Tessa. Tessa volunteers, but Jem refuses and hurls both the letter and the drug into the fire. Will plunges his bare hands into the flames to save what he can, badly burning himself. That evening, Jem plays Tessa a violin piece he composed for her, and they agree to marry immediately.

Before the ceremony can take place, automatons attack the Institute. Mrs. Black, who once held Tessa captive, reappears with her severed head mounted on a mechanical body. She seizes Tessa and forces chloroform over her face. Jem chases the fleeing carriage but collapses. Jessamine Lovelace, a former Institute resident who betrayed the group to Mortmain, is mortally wounded and dies in Will's arms. Her last words reveal that Will is "a terrible Welshman" and that Mortmain is "in Idris."

Cecily deciphers the clue: Jessamine's reference to "Idris" points not to the Shadowhunter homeland but to Cadair Idris, a mountain in Wales near where the Herondales grew up. Consul Josiah Wayland, the Shadowhunters' political leader, dismisses the theory and forbids Charlotte to act. Jem, gravely ill but lucid, reveals he knows Will loves Tessa, forgives him, and insists Will ride after her with his blessing. Will says what he believes is a final farewell and rides out alone.

Tessa wakes in the carriage with Mrs. Black, who tells her she was "created to be the ruin of the Nephilim," the Shadowhunters' angelic name for themselves. Tessa hurls herself from the speeding carriage, but her clockwork angel necklace expands, sprouting wings, and bears her gently to the ground. She disguises herself using her shape-shifting power, but the pendant gives her away and automatons recapture her.

Inside the mountain stronghold, Mortmain reveals Tessa's origins. Her mother, Elizabeth Gray, was unknowingly a Shadowhunter, switched at birth by faeries. Because Elizabeth bore no runes, she could carry a child fathered by a Greater Demon, a powerful demon Mortmain sent in her husband's form. Tessa is half-Shadowhunter, half-demon, a combination thought impossible. Mortmain forces Tessa to Change into his dead adoptive father, the warlock John Shade, whose spirit overwhelms her and writes down the binding spell needed to animate automatons with demon souls. The resulting automatons are invulnerable to seraph blades, the Shadowhunters' angelic weapons.

On the road, Will is struck by agonizing pain as his parabatai rune bleeds and fades. He believes Jem has died. Grief-stricken, he presses on to Cadair Idris, where the glacial lake pulls him down into Mortmain's underground stronghold. He finds Tessa imprisoned behind a magical barrier and rushes to her, unknowingly crossing the barrier and trapping himself as well. He tells Tessa that Jem is dead. Believing they will die the next day, they confess their love and spend their last hours together.

Meanwhile, Charlotte bypasses the Consul and appeals to the full Clave, the Shadowhunters' governing body. Gabriel, who had been pressured by the Consul to spy on Charlotte, publicly confesses and declares his loyalty. His brother, Gideon Lightwood, proposes to Sophie Collins, the Institute's maid, and she accepts. No other Shadowhunters answer Charlotte's call, but three Silent Brothers volunteer. Henry Branwell, Charlotte's inventor husband, has completed a Portal, a device for instantaneous travel, with help from Magnus Bane, a warlock ally. The rescue party steps through to Cadair Idris.

Magnus dissolves the barrier trapping Will and Tessa. In the vast chamber of automatons, Henry accidentally triggers one; the creature breaks his spine, paralyzing him permanently. Battle erupts. Will fights beside a hooded Silent Brother and realizes with shock that it is Jem, alive. Jem underwent the Brotherhood's transformation at the moment of death, the only way to survive. His parabatai rune faded because of the ritual, not death.

As the Shadowhunters are overwhelmed, Mortmain gloats from above that his automatons are simultaneously attacking the Shadowhunter Council in the homeland of Idris. Tessa, seized by a demon, reaches inward to her clockwork angel and touches the imprisoned spirit of the angel Ithuriel. She Changes into the angel, an unprecedented transformation, blazing with divine fire. She crushes Mortmain, and his death instantly deactivates every automaton. The angel's fire scorches through Tessa, and she collapses unconscious as Ithuriel's spirit is freed.

In the aftermath, Tessa lies delirious with fever for days. Henry survives but will never walk again. The attack on Idris killed the Consul and many others before Mortmain's death halted the machines. Jem, now Brother Zachariah, is permitted one visit. He tells Tessa he can never marry but promises to meet her annually on Blackfriars Bridge, then visits Will and urges him to marry Tessa with his blessing. The Inquisitor, a senior Shadowhunter official, offers Charlotte the consulship, which she accepts. Sophie undergoes the Ascension, becoming a full Shadowhunter by drinking from the sacred Mortal Cup. Will courts Tessa, and on the snowy steps of the Institute at Christmas, he proposes: "Marry me, Tess. Marry me and be Tessa Herondale." She accepts.

The epilogue spans decades. Tessa, immortal and unchanging, narrates her life with Will: years of marriage, children, growing old together while she remained young. Will dies in 1937 with Tessa at his side and his hand in Jem's, as Jem plays his violin one last time. For over 130 years, Tessa meets Jem annually on Blackfriars Bridge. In 2008, Jem appears fully cured, no longer a Silent Brother, freed by a recent Shadowhunter crisis. Tessa tells him she has loved him all this time. He nearly walks away, fearing she cannot love him after her lifetime with Will, but she follows and asks him to see the world with her. They kiss on Blackfriars Bridge as the story comes full circle.