first contact between earth and Aliens in this universe happens when people from HomeWorld come to the Earth trying to discover the place of origin of thier people

they come to the Earth because the origine of Terrans on HomeWorld was that Carston Hawthorn AKA Zenneth planted a colony there by traveling back through time. 

the arrive in Mexico and get involved with a group of people who include the father of Lonnie Orlando Cartwright , who Jordan Keerigan-Smith and Suzanne Patton search for in Mexico in the Lynzee's Fountain story