Clockwork Princess is the third novel in The Infernal Devices Series by Cassandra Clare. In this novel, Tessa is engaged to marry Jem. Jem’s illness flares up after a battle with a demon, causing them to move up the wedding date. Unfortunately, the wedding is permanently delayed when Tessa is kidnapped by her arch nemesis, Mortmain, and Jem makes the difficult choice to become a Silent Brother to save his life. Clockwork Princess is an exciting ending to the trilogy that completes Ms. Clare’s series, The Mortal Instruments.

Tessa is trying on her wedding dress when Gabriel Lightwood arrives at the London Institute and announces his father has turned into a worm. Will informs everyone that Benedict Lightwood has been suffering from the demon pox and the worm is the type of demon he has become. Tessa goes with Gideon and Gabriel Lightwood, Will Herondale, his sister Cicely, and her fiancé Jem Carstairs to Lightwood House. As Will and the others fight the worm, Tessa takes charge of the Lightwoods’ sister. As things resolve, Tessa joins Henry in Benedict’s study where they find writing on the wall that refers to the Infernal Devices, automatons used by Mortmain.

When the battle is over, and Gabriel has killed his father, Jem collapses. Everyone, Will especially, are shocked to discover that Jem has been using his medication, the yin fen that he is both addicted to and which is the cause of his inevitable death, in large doses and is nearly out. Will goes to buy some more, but discovers someone has bought up all the yin fen in London. Without it Jem will die. Jem tells Will that he can try to find a cure. For this reason, Will and Tessa go to visit the wizard, Magnus Bane, in hopes that he might have some clue as to where they might find a cure, or at least more yin fen.

As Jem’s illness worsens, Tessa begins to fear he will die before they can marry. Therefore, when Mortmain sends some yin fen and says that he will send more in exchange for Tessa, Tessa agrees. Jem will not let her, however, and tells everyone he would rather die than let her go to Mortmain. The choice is taken out of Tessa’s hands the next day, unfortunately, when a group of automatons show up at the institute and kidnap her.

Jem collapses after Tessa is kidnaped. Will is torn between his need to go save Tessa and his need to remain with Jem. Jem settles the matter when he overhears Will discussing with Magnus the fact that Will is in love with Tessa. Jem tells Will that if anyone else should be with Tessa it is Will. Jem then sends Will to find Tessa and save her from Mortmain.

As Will travels to Wales where he believes Mortmain has Tessa, he feels the moment when the parabatai tie between he and Jem is broken. With this knowledge, he goes to Tessa and tells her Jem is dead. Will and Tessa consummate their love for one another and are surprised the following morning when Magnus finds them. Magnus and Henry have created a portal that allows the Shadowhunters from London to quickly travel to Mortmain’s stronghold in Wales. Unfortunately, Henry touches something on the automatons they have discovered and causes them to come to life. Henry is badly injured in the subsequent fight.

One of Mortmain’s automatons takes Tessa during the fight. As Mortmain stops the actions of the automatons, he calls for Tessa. Aware that Mortmain plans to kill her friends and force her into marriage, Tessa uses her shapeshifting abilities to turn into an angel. This angel kills Mortmain, but it also leaves Tessa very ill. When they return to the institute, Will sits by Tessa’s side and becomes convinced that the bond between she and Jem needs to be broken. For this reason, Will asks Charlotte to summon Jem who has by this time begun his transformation into a Silent Brother.

Jem comes and tells Tessa that he loves her, but he knows that Will loves her as well. They discuss the situation and decide that it is better that Tessa move on with her life with Will. They agree to meet once a year on the bridge in London where they first met. Jem returns to the Silent Brother city while Tessa and Will find their way to one another. Tessa and Will are eventually married and they live more than forty years as man and wife, including the birth of several children. As Will ages, Tessa does not. Eventually Will dies. Afterward, Tessa travels the world, only returning to London each year to see Jem. Then one year, Tessa is pleasantly surprised that Jem has finally found a cure for his illness and has left the Silent Brothers. At first Tessa is reluctant to love Jem only to watch him age and die as she did Will, but then realizes that she has continued to love him for more than a hundred years and cannot let this opportunity pass.