THX, or "TEX," is the household robot of Masaki Masakazu and his family. Initially, it remains silent except in emergencies, and the reason for its quietness is a mystery. Eventually, it is revealed that after THX was overheard offering counsel to young Masaki during his parents' divorce, Masaki's father gave a strict order forbidding the robot from speaking. A parent's command to their child takes precedence over all others, except when there is a threat to the child's or another person's safety. Years later, when THX detects a danger, it finally breaks its silence to warn Masaki about an infiltration android.