Mark investigates the disappearance of three scuba divers, two of whom are 'replaced' by waterbourne aliens (Tiffany Bolling as Lioa / Dilly and Burr DeBenning as Xos / Chazz). The aliens assignment is to scout Earth and check its defense capabilities. They also give the impression that Mark may also be one of them, specially bred for Earth infiltration, but this is not specifically confirmed.