The Brothers' War between Mishra and Urza, the two most powerful artificers on Dominaria, is coming to a head in the Argoth forest. Discovered accidentally by the pilot of a lost ornithopter, Argoth's rich natural resources soon prove irresistible to the brothers. Each attempts to claim those resources for himself to fuel his war efforts-and to keep the other from obtaining them.
The struggle for Argoth involves not only the brothers but also the sentient forest itself, with terrible consequences: Urza is pushed to the limits of his sanity, Mishra into the waiting arms of Phyrexia, and Argoth toward utter extinction as the three-way war drags on. In the apocalyptic final battle, Urza personally vies with Mishra and reveals him to be a construct of flesh and Phyrexian metal. When Mishra completely abandons his humanity to attack his brother, Urza releases an enormous burst of power that destroys both Mishra and a huge portion of the forest they were fighting over. At the heart of the maelstrom of energy he has created, Urza transcends his mortal origins and takes his place as a planeswalker, the most powerful kind of spellcaster, with the ability to travel between the planes of the multiverse.
Unbalanced by both his transformation and the catastrophic events in Argoth, Urza fixates on the beings responsible for his brother's physical and spiritual corruption: the Phyrexians. He uses his newfound ability to traverse the planes seeking out his enemies but is unable to find their home plane until he rescues Xantcha, a Phyrexian construct about to be executed by her kin for failing to realize her full potential. With Xantcha as his guide, Urza is finally able to planeswalk to Phyrexia; almost immediately, however, he finds that he has underestimated both his enemy and the terrain of the battle. Phyrexia is a noxious, corrosive place, and the formidable horrors that live there attack Urza upon his arrival. Assaulted by the poisonous atmosphere and hard-pressed to defend himself, Urza is forced to make a hasty withdrawal. Before they leave, though, Xantcha reclaims her personality matrix (held in a magical mechanism called her "heart") from her former masters. Wounded and ailing, Urza blindly flees from Phyrexia with Xantcha in tow, his mission a failure and the advantage of surprise squandered.
Urza's flight from Phyrexia lands him in the company of another planeswalker-Serra. She has sequestered herself in her own private plane devoted to healing, order, and purity. Urza and Xantcha are granted Serra's hospitality and spend five years recovering from their Phyrexian ordeal. Once he is restored, Urza recognizes some structural similarities between Phyrexia and Serra's realm that do not bode well for either plane. Artificial planes such as these, he discovers, eventually degrade and collapse. This fact explains the Phyrexians' aggression: their own plane will someday cease to exist, and they must find another, more stable place to live. Urza is alarmed at the realization that Dominaria is the Phyrexians' target and quickly returns home with Xantcha. Soon after, a Phyrexian assault force arrives in Serra's realm, having tracked Urza there. There is a short and devastating battle between the Phyrexians and Serra's defenders until the former realize their prey has escaped them. They abandon the fight and quit the plane, but not before inflicting heavy casualties, seizing prisoners, and forever spoiling the realm's pristine wonder.
Unaware of the damage done in his wake, Urza returns to Dominaria and attempts to organize resistance to the Phyrexian invasion. He has physically healed in Serra's realm, but is still mentally unbalanced from his experiences. Xantcha forces a confrontation and resolution so that Urza can bring his full faculties to bear on planning the resistance: she repeatedly exposes him to a young man who is virtually identical to Mishra. She succeeds in calming Urza's troubled mind, but they are soon attacked again-this time by the Yawgmoth praetor Gix. This attack kills Xantcha and the Mishra double, as well as Gix himself. Urza is able to recover Xantcha's heart and puts it away until he can find the proper use for it.
Urza becomes convinced that the Phyrexians attempted to invade once before and were driven off by an advanced but extinct race called the Thran. With a new clarity of purpose, he decides to travel back in time to meet the Thran and learn their secrets. Toward this end, he establishes a wizards' academy on the remote island of Tolaria and staffs it with the best and brightest mages he can find, all the while taking extraordinary steps to avoid infiltration by Phyrexian spies. As the mysterious patron of the academy, Urza personally verifies each applicant's identity and issues an island-wide ban on adult visitors. He guides all manner of time-travel experiments, and constructs a mechanical being who becomes his primary tool for gathering data. He incorporates Xantcha's personality matrix into this sentient silver golem, named Karn, so that it will learn and mature. The experimental time portal requires an awesome amount of power, yet the golem reports that he is able to go back in time a maximum of only two days. Urza presses on, however, feeding more and more power into his time portal until inevitably, it overloads and explodes, flooding the entire island with temporal distortions. Barely half the academy's population survives, and Urza is forced to abandon this approach to defeating the Phyrexians. In time the academy is rebuilt and continues to teach and train, but Urza departs Tolaria immediately after the disaster to search for more present-day information on the Thran.
In the volcanic land of Shiv, Urza discovers the mana rig, a huge installation full of functional Thran technology. After earning the respect of the saurian viashino who live in the installation, Urza explores the equipment and determines its purpose: to create huge crystal batteries of energy, called powerstones, that can be used to run all manner of artifact devices. Although he and the viashino cannot operate all the necessary machines on their own, Urza begins to see the advantages of this energy source. He envisions a conglomeration of artifice, organic components, and pure magic that, when powered by such a stone, could be used together to halt the coming invasion in one fell swoop.
Constructing such a complicated weapon will take decades, perhaps centuries; Urza begins to make the necessary preparations for designing, transporting, and storing each part of his new Legacy until he is ready to use it to save Dominaria.