Earth under siege from space. A professor assembles a team, builds three vehicles that lock together into one giant robot, and spends 39 episodes proving the concept works. Zero Tester is exactly what 1973 wanted from mecha: no wasted time on feelings, no second-guessing the mission, just the clean geometry of five pilots protecting the planet one Armanoid at a time. The vehicles transform because the threat demands it, not because the show needs character development. This is pure mechanical efficiency dressed up as a TV show.

A string of accidents in space turns out to be the doing of the Armanoids, aliens who intend to conquer Earth. Professor Tachibana brings together a team at the Future Science Invention Center and readies five advanced vehicles for Shin, Go, Lisa and Captain Kenmotsu. Three of these Tester machines join to form the giant robot Zero Tester, which drives back the Armanoid invasion. Early seventies mecha action and science fiction adventure, later continued under a new title with a fresh alien enemy.