On patrol in Star City, the Green Arrow is drawn to gunfire, coming from a factory. Through a dense haze of smog, the Green Arrow can just make out a lone watchman, Greg Sanders, fending off two assailants. The Green Arrow launches a flare arrow, illuminating the battle. With greater visibility, Sanders begins to gain the advantage over his assailants. The flare arrow, however, upon falling into the river, ignites the chemical laced water. The Green Arrow activates his emergency signal, summoning Superman, and the Green Lantern, to his aid. While Superman keeps the fire from spreading into Star City, the Green Lantern extinguishes the blaze at it's source.
Sanders calls out to the Justice League of America as they depart, but his calls fall on deaf ears. Superman, and the Green Lantern, take the Green Arrow to a rooftop in Star City. There, the Green Arrow steps into a transport tube, for the very first time, where he is teleported to the Justice League of America's new satellite headquarters. Sanders learns that the Justice League of America is making an appearance at a charity event. Sanders is attacked, again, by the same two assailants. Sanders shoots out one of the tires of their car, causing a collision with a light post. Unfazed, the two men continue their relentless pursuit of Sanders.
The Justice League of America formally announce that the Black Canary has joined their ranks. Sanders rushes into the charity event, his pursuers hot on his heels. The Black Canary leaps to Sanders defense. Sander's assailants open fire, but their attack is blocked by the Green Lantern. The Black Canary, and the Batman, subdue Sander's assailants. Suddenly, Superman throws his invulnerable body on top of the would-be killers, as they explode. Sander's assailants are revealed to be automatons. Sanders takes possession of their weapons. Sanders is brought to the Justice League of America's satellite headquarters.
Sanders reveals that the factory he has been working for, as a watchman, is actually purposefully polluting Star City. Sanders has been on the run, ever since he stole several files from the factory. Sanders reveals that he was once the costumed crime fighter known as the Vigilante. The factory papers turn out to be chemical compositions of various pollutants, and a star map of the Sirius system. Superman, and the Green Lantern, carry the investigation to Sirius. The Atom, the Batman, the Black Canary, and the Vigilante return to the fatory. The Green Arrow confronts the city manager of Star City. After lecturing the city manager, the Green Arrow is arrested and incarcerated.
Superman, and the Green Lantern, discover that the fifth planet in the Sirius system, Monsan, is completely devoid of life. Monsan's ecosystem and atmosphere have been fatally altered by a wide array of pollutants. The Justice League of America, and the Vigilante, upon arriving at the factory, are immediately attacked by a group of automatons. Unbeknownst to the four heroes, they are under observation by the alien Doomsters. Suddenly, the automatons' weapons, that the Vigilante is now brandishing, dissolve into sleeping gas. Subdued, the Justice League of America, and the Vigilante, are thrown into a steel net, and slowly lowered into a vat of boiling acid.
This story was originally published in Mystery In Space #6 ( February-March, 1951). A plot summary for this issue can be found at the link.