Three robots that shouldn't work together become unstoppable when they have to. God Sigma isn't a mecha show about saving the world, it's about a martial artist figuring out how to lead two other pilots through an occupation that keeps getting worse, one Elda attack at a time. The combination gimmick matters because it forces the lead to trust people who aren't him, and in a 1980 mecha show where pilots usually solve problems alone, that's the entire story.

In 2050, Earth faces an energy crisis, and a gifted scientist in Trinity City creates Trinity energy, a power source strong enough to supply the whole planet. When the alien Elda Empire learns of it, they set out to take it by force. Their forces attack and occupy Earth's space colony on Io, a moon of Jupiter. Earth's defense rests with three giant robots, powered by Trinity energy, that combine into God Sigma. Its main pilot is Dantou Shiya, a martial artist who becomes a pilot. Armed with a powerful sword, God Sigma stands between Earth's peace and the Elda forces. As the fighting goes on, it grows into a long war to win back Io, told through the robot battles of an action mecha series.