Rhoma Grace is a 16-year-old student from House Cancer with an unusual way of reading the stars. While her classmates use measurements to make accurate astrological predictions, she looks up at the night sky and makes up stories.

When a violent blast strikes the moons of Cancer, sending its ocean planet off-kilter and killing thousands of citizens-including its beloved Guardian-Rho is more surprised than anyone when she is named the House's new leader. Then, when more Houses fall victim to freak weather catastrophes, Rho starts seeing a pattern in the stars. She suspects the exiled 13th Guardian of Zodiac has returned to exact his revenge across the Galaxy.

Now Rho must travel through the Zodiac to warn the other Guardians. But who will believe anything this young novice says? Whom can Rho trust in a universe defined by differences? And how can she convince twelve worlds to unite as one Zodiac?