This is book number 2 in the Continuum Trilogy series.
Reaching the surface was just the beginning.
As Myra Jackson and her friends set out to find the First Continuum, Captain Aero Wright and two companions from the outer space Second Continuum find themselves banished for treason and stranded on Earth. Wright has vowed to complete his late father's mission to recolonize their ancestral planet, but his true mission is to find the mysterious girl who haunts his dreams. Meanwhile, Myra and the young refugees of the underwater Thirteenth Continuum must make an unlikely ally if they are going to survive the hostile surface world and reach their destination, the nexus of humanity's hope for survival. As their paths begin to converge, the Beacons that guide and connect Myra and Aero begin to prove their power, and a shadowy force with a centuries-old grudge reveals itself.