The two arrive on the surface of Planet X in a borrowed spaceship. Traveling to another continent to protect the ship, the two are separated by five miles. Planet X resembles Earth in all other ways except for the lack of cities, people and significant life; same gravity, air density and water distribution.
Both are aware of each other but neither appears to move for ten, maybe fifteen minutes. Barry checks his calculations for the speeds he will need to affect Superman at all.
He sees the possible permutations required to get Superman into the right position to execute his attacks and sees Superman already countering more than half of them in less than a nanosecond. Thousands of choices, all failures.
How fast is Superman truly capable of being? One thousandth of a second, one ten thousandth? One millionth? Then he sees a possibility. Something Superman can't possibly know yet. The Speed Force crackles and the Flash disappears.
Superman's keen intellect has already spun up his mental capacity to sense the Flash. He knows the Flash will need to move so fast, even his superior senses will never detect him in time. He closes his eyes and waits. His every nerve tingling in anticipation. He feels the electromagnetic field of the planet flickering all around him. He won't look for him, he will feel his disturbance in the planet's magnetic and gravimetric field.
The Flash vanishes. The electromagnetic aura of the planet reverberates with his powers. The Flash believes himself to be invisible, but he affects the world the same way Superman does. At the fundamental level.
Feeling no immediate impact, Superman realizes the Flash needs running room, time to build up speed.
Superman can feel the buildup of energy approaching his position just ahead of the Flash. It took him three seconds to circle the planet. He's still way under lightspeed. He has to keep this battle just under the speed of light to take advantage of his IMP.
The first pass doesn't even draw near Superman. The Man of Steel marks the mental path and waits. Three seconds again. Why so slow? The Flash whizzes by again.
Why won't he attack, Barry wonders. This plan only works if he engages first. Maybe if I increase the threat level. Barry disappears in the distance and takes ten seconds to return.
Superman surprised at the length opens his eyes to a barrage of boulders crashing into him at near relativistic speeds. The shockwaves caused by their sonic booms split the sky, creating plumes of blinding dust. Swinging into action, the Man of Tomorrow deflects the smaller and faster particles of debris and dodges the slower, larger ones.
His motions have to be fast. Faster than he normally move on Earth, because if he misses, each hunk of granite explodes with the force of a small bomb. Turning the ground into near light-speed projectiles is clever. But Superman's reactions are equal to the task and each snap of his fist creates ground-splitting explosions of force. Each deflected hunk of granite becomes a fine cloud filling the air around him. His fists are a blur as the cloud of missiles continue unabated.
There's a pattern here. This isn't random. I can't figure it out. The cloud of debris soon envelops Superman.
The hail of missiles stops and the Flash appears, for just a second, striking Superman on the chest. Before Superman can react, the Flash is gone. I can't move, he thinks. What did he do to me? Where did he go? With the cloud of dust all around him, Superman tries his X-ray vision only to realize what has happened.
The dust is filled with lead. He never sees the Flash as he circles the world five times in the next second. Realizing the danger, Superman vibrates his own molecular structure trying to overload whatever effect was holding him in place. He's all around me, setting up for his attack.
Superman stops struggling and braces himself. The Flash takes his one shot, igniting the atmosphere around his fist as he reaches ninety nine percent the speed of light. He can see the cloud, he has already mapped the position and knows exactly where Superman is. To the inch.
The world vanishes from sight only his mental map of everything remains. The ground beneath his feet explodes as he passes over it once, twice, three times, four, five, crossing the oceans, he vaporizes tons of water behind him, creating clouds of steam that span the world.
His mass is increasing, he weighs as much as a truck on his first pass, an aircraft carrier on his next, a pyramid on his third, a moon on his fourth and a planet on his fifth. Earthquakes with every step.
Superman knows he's coming. It has only been a few seconds since he was immobilized. While he can't move, he can see. He has five nanoseconds before the Flash makes his final approach. More than enough time. His eyes glow a fiery red. He waits until the Flash has passed for the last time. He breathes in deeply, sucking in as much of the dust and debris in the air as possible.
Three nanoseconds. Even this tiny movement is taking all of his strength. He can feel the paralyzing effect faltering. Straining against it, he holds his breath as the granite dust sears even his invulnerable lungs.
One nanosecond. The Flash is faster than my heat vision. When he's running away from it. Superman expels the granite dust into a cloud directly in front of him.
The Flash arrived right on time, his Infinite Mass Punch delivered with perfection. The blow drove Superman into orbit. In that same moment, an explosion erupted at point blank range right as Superman achieved escape velocity. The Speed Force transferred the accumulated energy directly to the Man of Steel, shockwave and all. For the Flash, his blow, despite it's incredible payload is silent as the grave.
Superman's heat vision, ignites the rapidly expanding dust cloud with a temperature equal to the surface of the sun. An explosion fills the air around the Flash.
The Flash never saw it coming.
A dust explosion coupled with his tangible state meant he caught the full brunt of the blast and landed several hundred feet away unconscious and battered. The Speed Force absorbed a great deal of the explosion but the Flash would not regain consciousness for hours.
Superman had been struck with the force of a million megaton bomb.
Barry had pushed his powers to the limit. 99.8% the speed of light gave him an incredible punch, a degree of superhuman durability and no chance to dodge the dust explosion. Any other normal man would have been torn to shreds.
The Man of Steel woke up, battered and in orbit, impressed by the ingenuity of the Fastest Man Alive.
An hour later, Barry Allen wakes with a splint on his arm and Superman watching him with a smile.
Barry scratches his head before remarking, "Dust explosion, huh? How'd you come up with that? I didn't think you knew much exotic science."
"You're right, I was inspired by my experiences as a farmer. Grain silos and dust explosions go together. I have to hand it to you. You are more powerful than I ever imagined. How did you do the trick with the rocks?"
"The same way I made you stand still, I imparted momentum onto them the same way I stole your speed, until you couldn't move at all."
"And the lead hidden in the dust cloud?"
"I stopped to mine. You didn't notice the five second gap in there?"
"I did. I thought you changed your mind about the fight. How did you know it would vaporize?"
"At those speeds it wouldn't be able to do anything but. I needed a cover you couldn't penetrate. Change my mind? Not a chance. Batman would never let me hear the end of it. I have to tell him his idea worked."
"I knew he had to be involved."
"No, the plan was mine. He just helped with the calculations. I needed to know how much force to use so I didn't accidentally kill you."
"And he knows how much force that is?"
"Never doubt it for a second. The man sometimes gives me the creeps."
"Me too. Race you back to the ship."
"You're on. You know you can't win, right?"
"Even Superman's gotta have a dream."