Corinna is eleven-years old when she discovers that the cupboard under the sink is actually a portal to another world, the Land of Nibiru. There, a war is raging between the Zonnier Hordes and the Resistance, assisted by the High Priests. When she tumbles out of the Portal, she immediately sees the Hordes and warns the Resistance, saving many lives and earning her the title of Chosen One, or Princess. For a year in Nibiru, she is celebrated, prayed to, and educated in the art of swordplay and stealth.

Now, however, Corinna is being returned to her own world by the High Priest. He tells her she will be safer there, but that he will return to fetch her in a week, at most. He tells her not to move, and zips back through the Portal. Corinna assumes her parents will be happy to see her after such a long time, but when life proceeds as usual, she realizes no time at all has passed on her side of the portal.

As time passes, the High Priest doesn't return. She refuses to go too far from the house, worried she'll miss him. She stops going to school and is homeschooled instead. When her parents divorce, she chooses to live with her mother in the house, even though she prefers her father. Years pass. Corinna keeps Nibiru alive by drawing detailed sketches. When she turns seventeen, Corinna gets pregnant and doesn't attend college. Soon she has four children, and the father leaves her. Still she waits. She starts a new job as an archivist. She soon can barely remember if Nibiru was just a dream.

She dreams of Nibiru for the first time in years, and soon, is dreaming about her experience every day. Through her dreams, she comes to a revelation: the High Priests weren't really helping the Resistance. In fact, the High Priests had poisoned the land of the Zonniers, leaving them starving and with no place to go. The Resistance and the Zonniers had no reason to fight each other really, and the High Priests were only stoking the flames. On her forty-ninth night of dreaming about Nibiru, she encounters Cairn, a friend from that other world, who gives her the long-sought-after circlet that is supposed to belong to the Chosen One. When Corinna wakes up, she has it in her hand.

One day, her boss at work tells her a strange man has been taken into police custody and is asking for her. She knows it is the High Priest, but doesn't go to him. That night, she discovers she is able to open the Portal with the power of the circlet. Opening it, she and her four children go through and enter Nibiru once more, stopping a raging battle. She convenes the remaining High Priests and brings them through the portal into her house, where she leaves them and tells them she will return in a week. But she does not return. For the rest of her days, she and her children reign over Nibiru, reconciling the Zonniers and the Resistance, and establishes peace throughout the land. The High Priests never return.