Still reeling from his last defeat at the hands of the League, space pirate Kanjar Ro (from the two-part animated episode “In Blackest Night”) returns to collect some of the deadliest weapons in the galaxy...weapons he’s used to turn the World’s Greatest Super Heroes into the world’s most powerful slaves!
There exists a sector in space where three rulers have agreed to a battle which allows them to deploy three soldiers each and limited weaponry.
On one such event, Hyathis' army has won a Gamma Gong and are taking it home, when Kanjar-Ro invades their spaceship, and enslaves them. He is secretly in alliance with the Mechanoids and the Reptilians.
Hyathis informs the Justice League and seeks their help. The League goes to investigate the ship, and they too are enslaved by Ro. Ro then enters Hyathis' world and enslaves the whole capital, while ordering them to give him all their weapons.
Jo'nn Jo'nnz cleverly overcomes the effects of the Gamma Gong and mobilizes the whole Justice League. Ro senses trouble and is about to flee when he is offered a solution by Batman. He can become one of the Leaders. Ro is overjoyed because his dreams have come true.