Office drone Edward Borman is spinning his wheels in a dead-end office job until, one night, his drudgery is interrupted by the appearance of a glowing figure who blasts the janitor with a beam of energy. Borman is close to being zapped, himself, when a mysterious goggled figure appears and shoots it out with the incandescent invader. Borman, caught in the crossfire of a heated battle he doesn't understand, cowers in the corner of a hallway, only to find himself face to face with the alien. Fortunately, he's saved at the last minute by a door kicked down by the second arrival, who removes his goggles to reveal a face entirely human.
Rescued from alien annihilation, office drone Edward Borman finds himself at the mercy of his savior, who quickly pulls him into hiding as two more glowing creatures appear over the liquid corpse of their fallen brother. A flash of light, and the mess is gone, at which point the two creatures flee into the office. "What are they," asks Borman. His answer? "Dangerous. Very dangerous." A tense game of cat and mouse ensues, with Borman's pistol-wielding protector leading him on an escape from the intruders. In a construction zone, the duo witness a worker being zapped, and are forced to descend to the building's parking garage. There, they see two metallic humanoids working on a device while the glowing creatures patrol. Yaeger reveals that the contraption in question is a "Gravity Engine," and deduces that the building is being used as a magnifier. In a quiet moment, Yaeger explains things to Borman: The invaders are men from Mercury, made of light so dense, it has become solid. Using some nifty technology, he deduces the Mercury Men's plan: To pull the moon into the Earth.
Unable to accept the truth being told him by Jack Yaeger, Edward Borman is about to call the police when the sound of the Mercury Men's approach tells the two that danger is on its way. A Mercury Man appears, but is quickly dispatched by Yaeger, who leads Borman to the roof to contact the mysterious League. Unfortunately, the news isn't great, as Yaeger learns the League can't intervene for several hours. But just when things seem lost, a message comes through from Doctor Tomorrow...with a message for Borman! But just when the League is about to send through instructions on reversing the effects of the gravity engine, the room starts to shake. The engine has been switched on. Borman flees - only to find himself face to face with one of the aliens.
Trapped in an elevator with a Mercury Man, Edward Borman fights for his life. As the creature attempts to choke Borman to death, the door opens, and Yaeger intervenes. A fistfight ensues, with the Mercury Man quickly gaining the upper hand. Fortunately, Borman is able to find Yaeger's pistol and, even without his glasses, gets it to the Captain, who shoots his attacker dead. In the aftermath, Yaeger and Borman have it out, with the latter hoping to escape rather than help his rescuer shut down the gravity engine. A heartfelt speech from Yaeger, however, convinces Borman that there's more at stake than one man's life. The pair head up to the top floor to seek the source of a signal that's leading the Mercury Men, and find it: A brain in a jar.
The moon, being pulled by the gravity engine, drifts slowly toward Earth. Meanwhile, Yaeger and Borman consider the brain in the jar. When Yaeger goes to approach it, a voice comes through his radio - it's the brain, and it knows his name. It reveals there's a puppet master even higher up the ladder, and that it's aware of his previous conversation with the League. It then sends Mercury Men after Yaeger, who takes Borman into an office and hides him before heading out to face his fate...which, from the sound of it, is pretty bleak.
After hearing blasts that seem to signal Yaeger's capture, Borman emerges from his hiding place and begins to seek out a way to defeat the Mercury Men. He uses Yaeger's radio in an attempt to contact the League, but has no luck. But the radio's static is interrupted by the sound of jaunty music. Following the sound, Borman finds a TV broadcasting what at first appears to be a test pattern, but which then resolves into a face; the face of Doctor Tomorrow. The League's commander asks Borman to free Yaeger and destroy the gravity engine. Borman hesitates, but Doctor Tomorrow uses man's monumental achievement in traveling to the moon to inspire his charge.
The Mercury Men are torturing Jack Yaeger in an attempt to locate the transponder beacon, which he'd left with Borman. Meanwhile, Borman sneaks in, using his staple remover to attempt to untie the good Captain. The brain threatens Yaeger with the torture of his best girl, which only serves to enrage the heroic engineer. He doesn't crack, but the brain is able to read Yaeger's thoughts and discovers the truth. But Borman has already freed Yaeger. In one swift movement, he retakes his pistol, dispatches the brain's servants, and makes good on his threat to crush the sinister cerebellum with his bare hands. In a moment of quiet, Yaeger explains that the Mercury Men's attack is motivated by fear of humanity's ability to leave their planetary cradle. He determines that, in order to save the Earth, they have to not just shut down the gravity engine, but reverse it...which means they have to get close. Just when it seems it might be hopeless, Borman comes through with a plan.
Yaeger dons the suit of the dead Mercuy Man from the elevator and outlines the plan for Borman - while he reverses the main engine, Borman needs to reverse the polarity on the three conduits that connect it to the building. The plan in place, the two get to work, with Yaeger threading a gauntlet of invaders to approach the engine. As he gets to work, Borman flips the polarity on the first conduit. The second conduit is flipped, and all seems to be going according to plan. As Borman approaches the third conduit, however, he encounters a lone woman, whose scream attracts the attention of the glowing sentries. She introduces herself as Grace, and Borman attempts to help her escape observation. But the delay leads to problems, as an alarm on the engine begins to sound, alerting the real Mercury Men to Yaeger's work. A battle ensues, with Yaeger emerging victorious. He now has a chance to work on the engine...but is suddenly alerted to the approach of the new sentries.
As the Mercury Men approach, Yaeger sets up a barricade and entrusts Borman with his pistol, which he finally finds the courage to use after cowering through a few salvos. The sentries seemingly retreat, but then more appear, and the attack intensifies, with Yaeger and the glowing invaders exchanging fire. Meanwhile, Yaeger rushes to make the final calculations for his alteration of the gravity engine. He flips a switch, but rather than reversing, the engine simply shuts down. It needs power, but as Yaeger goes to retrieve his pistol from Borman, he's gunned down by a lightning blast from one of the Mercury Men. Borman finds the courage to counterattack, but the pistol is now empty. Left with no alternative, Borman runs, but finds only a dead end. The moon is getting dangerously close to the Earth, now. Borman, seemingly hiding behind a pole, comes under fire again...but it was actually a clever ploy, as the aliens' lightning actually powers the gravity engine, causing its effect to reverse. As the moon drifts back into place, Borman flees another Mercury Men attack, and is saved as agents of the League appear! They take care of Yaeger and wrap things up, as Borman returns to his office, where all returns to normal.
The League reviews the surface of Mercury, seeing several structures. A mysterious figure is told that "He's arrived," before we cut to the planet Mercury in the year 1976. Jack Yaeger is investigating the Mercury base that served as a staging point for the Earth invasion we've witnessed in previous episodes. As Yaeger and a League colleague talk, we learn more about the villainous Chief Designer and his workings. The conversation continues, but then shots are fired, and Yaeger suddenly faces a surprise attack from a masked sniper. Using a projector, he's able to get the drop on his attacker, who he dispatches with a headshot. Investigation of the corpse reveals a journal with details on Yaeger...and pictures of Edward Borman. Back at his office, Borman receives an envelope from "Buck Rogers." When he opens it, bullets for the Lumiere pistol fall out...but it may be too late, as another assassin appears behind him.
