Superman is summoned to an emergency meeting of heads of intelligence in the C.I.A. Headquarters where he is debriefed on Black Zero, a mysterious alien saboteur who threatens to wipe out mankind. Secret agent 009 was about to transmit his hideout's location when he was caught and incinerated by Black Zero.
Superman is vowing to find Black Zero no matter what, when his super-hearing picks up a double heartbeat, right in that room. Superman attempts to lure Black Zero out by daring him to attack him, but he is seized by two giant flaming hands, one of which writes on the wall the message that humanity will be destroyed in three days.
Chief Inspector Watkins from Scotland Yard wonders whether he will be right, and Superman decides to prove Watkins wrong by taking him to his Fortress of Solitude, where he will use his super-computer to track Black Zero down. To his shock, Superman finds out that Black Zero has killed Inspector Watkins and taken his place.
Black Zero shifts back to his real form, and summons a flaming hand to prevent the Bottle City of Kandor from helping Superman. Superman demands to know how Black Zero can speak Kryptonese, and Black Zero reveals he is a saboteur from a space pirate empire, who was sent to Krypton several decades ago to destroy the planet before it developed space-faring technology and became a threat. Nonetheless, Black Zero heard about Jor-El's disregarded warnings and decided to investigate them. Black Zero found that, indeed, a chain reaction had started at Krypton's uranium core...though it was dying down by itself, so Black Zero decided to do Jor-El a "favor" and use a drilling rocket to reach the core and re-start the chain-reaction.
After listening to Black Zero's boasting and gleeful confession, Superman bursts out in rage. He cannot kill Black Zero, no matter how justified, but he can think of a better punishment for him. Nonetheless, Black Zero becomes intangible and slips through the wall, gloating about the incoming planet-sized fireworks. Superman is wondering how he can find Black Zero in time to save Earth, when he receives a telepathic message from the Phantom Zone: the Zoners have witnessed the scene and want revenge on Black Zero, too. Their leader Jax-Ur claims he can help Superman find Black Zero's hideout, and so that Superman knows Jax-Ur is not trying to trick him into setting him free, he swears by the Colossus of Hadrad (a sacred oath to Kryptonian criminals) that he will not betray Superman if he is released from the Zone.
Tropes:
- Abandoned Warehouse: Black Zero uses an old, abandoned movie studio as his hideout.
- Aborted Arc: It is revealed that an unnamed interstellar pirate empire plotted and helped cause the destruction of Krypton, having previously destroyed a number of civilizations which could pose a threat if they were allowed to develop space-faring technology. At the end of the story, Superman wonders whether he will ever be able to find and punish the rulers of the space pirate empire, but nothing came of it.
- Antimatter: Black Zero tries to annihilate Earth by dropping an anti-matter bomb on it. Superman doesn't dare to interact with it because it is primed to explode anyway, scattering its fallout across the planet.
- Apocalypse How: Black Zero attempts to blow Earth up by dropping an anti-matter bomb on it. Even if Superman tried to detonate the bomb in space, it would still scatter anti-matter fallout across the planet, killing everything.
- Apocalyptic Log: CIA agent 009 is caught and murdered by alien saboteur Black Zero as recording his findings on a tape.
- Combo Platter Powers: Black Zero can shape-shift, turn himself intangible, create and manipulate fire and transmute matter.
- Death from Above: When Superman saves Earth from his anti-matter bomb, Black Zero forms a giant hammer to drop it on Metropolis from his ship in retaliation.
- Desecrating the Dead: After Black Zero has been turned to stone, Superman wants to place his petrified figure among his enemies' statues. However, Jax-Ur shouts that Black Zero doesn't deserve being respected, or having his statue standing next to Jax's one (not to mention someone might find a way to restore him), so he smashes Black Zero's statue to bits.
- Digging to China: Black Zero intends to annihilate Earth using an anti-matter missile. Superman cannot risk to manipulate it, so he digs a wide tunnel through Earth so that the missile goes through the planet without touching it.
- Earth-Shattering Kaboom: Black Zero intends to blow Earth up with an antimatter bomb.
- Enemy Mine: Superman and the leader of the Phantom Zoners Jax-Ur work together to find and defeat Black Zero, an evil alien who intends to blow Earth up several decades after helping bring about Krypton's destruction.
- Evil Gloating: Black Zero cannot stop himself from ranting about his evil plan, gloating about his powers and smugly challenging Superman to stop him. Not only does the villain waste a perfect opportunity to kill Superman because Black Zero wants him to suffer, but he also gives Superman a clue to find his planet-shattering bomb.
- Fate Worse than Death: Superman stated he couldn't kill Black Zero but had a far worse punishment in mind. After Black Zero has been accidentally turned into stone, Superman reveals he intended to throw him into the Phantom Zone, where he would be plagued for eternity by Kryptonian criminals. Upon listening this, Jax-Ur rues the loss of his opportunity to torture the genocidal alien forever.
- Fleeting Demographic Rule: Through the ages there have been a large number of villains claiming responsibility for the destruction of Krypton: Klax-Ar in Superboy (1949) #67 (1958), Raspor in Action Comics #338 The Villain Who Married Supergirl (1966), Black Zero in this story (1968), -possibly- Brainiac in Superman: Brainiac (2008), Rogol Zaar in The Man of Steel (2018)...
- Forced to Watch: Black Zero shoots a Kryptonite bullet at Superman, and immediately telekinetically guides the bullet away from Kal-El because Black Zero wants his enemy to live enough to watch Earth explode by his hand.
- Forced Transformation: A Red-Kryptonite bullet causes Jax-Ur to change successively into a giant man-faced python, a massive rattlesnake and a scaly, snake-haired Medusa.
- Good Is Not Soft: Superman wants to kill Black Zero when he learns his role in the destruction of Krypton, but Black Zero just laughs his death threats off, since Superman has a code against killing. Superman retorts he has a far worse punishment for Black Zero in mind.
- Hoist by His Own Petard: Black Zero shoots Jax-Ur with a Red-Kryptonite bullet, expecting him to suffer some disabling or debilitating change. Instead, Jax-Ur starts transforming into giant reptilian or reptile-related creatures which let him fight Black Zero, culminating in a snake-haired medusa whose glare turns him to stone.
- I Gave My Word: When Jax-Ur offers Superman assistance to find and capture Black Zero, and Superman asks for an assurance that his old enemy is not trying to trick Superman into letting him out of the Phantom Zone, Jax-Ur swears by the Colossus of Hadrad (a sacred oath to all Kryptonian criminals) to not betray Superman if he is released from the Zone. After being released, Jax-Ur reiterates he will keep his word.
- Immune to Mind Control: Superman and Jax-Ur use bright, flashing lights to try to hypnotize Black Zero into revealing his secret plan to destroy Earth. However, Black Zero cannot be hypnotized, because his planet's surgeons placed an anti-hypnotic covering around his brain in order to prevent this.
- It's Personal: After learning that Black Zero made sure that the instability in Krypton's uranium core triggered its destruction, Superman declares that stopping and punishing the evil alien is a personal thing now.
- Just Before the End: Superman cannot seem to find a way to prevent Black Zero's anti-matter bomb from blowing Earth up within one hour, so he changes back to Clark Kent and heads towards the Daily Planet to spend his final hours together with his friends.
- Kill and Replace: Black Zero murdered and replaced Inspector Watkins of Scotland Yard to infiltrate a meeting of intelligence agencies in the C.I.A. Headquarters.
- Killed Mid-Sentence: CIA agent 009 is caught by Black Zero as recording his findings on a tape, and incinerated as he is revealing Black Zero's hideout's location.
- Killed Off for Real: Jax-Ur smashes Black Zero's petrified remains to bits in order to ensure that nobody may find a way to bring him back to life. And indeed, Black Zero has remained dead since 1968.
- Lizard Folk: A Red-Kryptonite bullet causes Jax-Ur to change progressively in different varieties of reptilian humanoids, from giant human-faced snakes to a scale-skinned, hand-clawed, snake-haired Medusa.
- Medusa: Jax-Ur is transformed by Red Kryptonite into a male Medusa: his skin becomes scaly and brown, he grows fangs, claws and snake hair, and one look at his face will automatically turn anybody to stone.
- No-Sell: Superman attempts to detonate Black Zero's anti-matter bomb safely in the space, trusting his invulnerability to keep him alive. The ensuing, massive shockwave is more powerful than anything Superman has felt before and blows all nearby asteroids to dust, but he survives.
- Not Enough to Bury: A CIA agent codenamed 009 tried to spy on saboteur Black Zero and got caught. To Superman's horror, an urn of ashes is all that's left of 009.
- Orbital Bombardment: Black Zero attempts to obliterate Earth by dropping an antimatter bomb on it. When his bomb fails thanks to Superman, the evil alien attempts to flatten Metropolis in retaliation.
- Personalized Pledge: Once upon a time, a criminal Kryptonian called Taz-Ki escaped the police by hiding into the Colossus of Hadrad, a huge stone face carved by natural erosion on a mountainside. Since then, all Kryptonian outlaws swear by the Colossus of Hadrad, considering it a sacred oath which no criminal would dare to break.
- Portent of Doom: The Book of Daniel's imagery is deliberately used when Black Zero creates a flaming hand to write on the walls of the C.I.A. Headquarters a message that the human race will be destroyed in three days.
- Race Against the Clock: Superman has three days to discover the secret of Black Zero and prevent the destruction of Earth.
- Reptiles Are Abhorrent: Villain Jax-Ur transforms into different kinds of reptilian or reptile-adjacent creatures after being hit with a Red Kryptonite bullet: a massive man-headed python, a giant rattlesnake, and a scaly, serpent-haired Medusa.
- Riddle Me This: Black Zero gives Superman a clue to stop him, as gloating he will never figure it out: "The Earth-bomb that will destroy this world is located in the emptiest place known". Superman checks on dried-up seabeds, poisoned valleys and dead deserts, finding no sign of bombs. Then, Superman realizes that Black Zero didn't say the "emptiest place known" was on Earth. Superman flies off world, and finds his enemy's spaceship stationed near from the planet.
- Rubber-Forehead Aliens: The CIA's officials deduce that the mysterious saboteur Black Zero may look human but he must be an alien agent because he lacks fingerprints, has purple eyes, and his tongue is forked.
- Taken for Granite: Black Zero gets turned into a stone statue by Jax-Ur, who had been transformed by Red Kryptonite into a Medusa-like creature with a petrifying leer.
- Thou Shalt Not Kill: After Black Zero -who intends to blow Earth up- has confessed to ensure that the instabilities in the Krypton's core led to its explosion, Superman declares that Black Zero must pay for his genocidal act with his life...before backpedaling because he cannot break his code against killing (so that he will come up with a far worse punishment).
