The detonation of the French nuclear bomb showered the iguana egg in fallout... Causing the foetus within to die, the intensity of the radiation disrupting the fragile development of the creature.

But assuming that didn't happen.

As the egg began to hatch, the misshapen and mutated iguana discovered it had seventeen eyes, and zero lungs. It promptly died.

But assuming that didn't happen.

The creature hatched, seemingly normal, but within a few months, was basically just a walking mass of tumours. It slowly died.

But assuming that didn't happen.

As the mutated lizard grew, its origin as an iguana proved disastrous, as it starved to death unable to eat enough algae to survive. 

But assuming that didn't happen.

As an ectothermic animal, it could not handle the cold very well, and being so large, lost heat very quickly. It froze to death swimming for too long.

But assuming that didn't happen.

As an ectothermic animal, it could not handle the cold very well. It had to lounge around in the sun to warm up. It was seen by a passing aeroplane. Bricks were shat. Bombs were later dropped. It was dead as hell.

But assuming that didn't happen.

As the mutated lizard grew and swam around, it attracted the attention of a passing US Navy warship. Upon seeing a giant lizard, the captain ordered his crew to sink the crap out of it. They did so.

But assuming that didn't happen.

As the mutated lizard grew, it grew too large to actually support itself, so that when it stepped on dry land, its legs snapped like twigs and the creature collapsed in a bleeding heap.

But assuming that didn't happen.

The lizard was right in the path of another nuclear test. The heat of the blast, sadly, reached temperatures in excess of millions of degrees for a few nanonseconds. Not long, but long enough to boil the creature's blood and vaporise it.

But assuming that didn't happen.

The lizard somehow survived the extreme heat... but not the added radiation, which even in an already mutant creature, was lethal.

But assuming that didn't happen.

The lizard somehow survived the extreme heat... And radiation... but the blast force was so powerful it ripped it in half.

But assuming that didn't happen.

The lizard somehow survived the extreme heat... And radiation... And being torn to bits... but was instead hurled away by the shockwave, landing on its head, and dying virtually instantly as his brain was fused into his arsehole via the long way.

But assuming that didn't happen.

The Worm Guy, Dr. Niko "Nick" Tatopoulos, researched his worms... and that was it. The lizard was dead as hell already and no one went looking for him. He was later eaten by a giant worm. It was how he wanted to go.

But assuming that didn't happen.

The lizard began to migrate, searching for a place to nest. Or rather, it didn't, as iguanas do not work that way, irradiated or not.

But assuming that didn't happen.

The lizard approached the shore... of Honolulu, as Hawaii is much closer and fits every other reason for going to New York.

But assuming that didn't happen.

The lizard approached the shore... of Tokyo, as unlike the script writers it was a traditionalist, damn it!

But assuming that didn't happen.

As the creature made its way to New York because reasons, it was detected, and promptly depth-charged to depth. Er, Death.

But assuming that didn't happen.

When it arrived in New York, it was immediately arrested and deported for not having a valid visa.

But assuming that didn't happen.

When it arrived in New York, it trod on some things then slipped on a yellow cab. They are, in fact, made of banana skins, for just such an occasion.

But assuming that... Wait, what?

Just messing with you, it actually stomped around for a bit and then they set up the fish trap... which didn't work as it ate algae.

But assuming that didn't happen.

The large pile of algae was covered by numerous tanks, heavy weapons, airborne gunships, and predator drones - no infantry armed with mere 5.56mm assault rifles, token grenade launchers, and M270 MLRS launchers being used as direct-fire weapons. Just lots and lots of nasty bang-bang.

The lizard was not an idiot, saw them all, and decided to just get the hell out of there.

But assuming that didn't happen.

The large pile of algae was covered by numerous tanks, heavy weapons, airborne gunships, and predator drones. They shot the crap out of him.

For good measure they also shot the giant worm, which had come along too.

But assuming that didn't happen.

The lizard escaped the attack, chased by AH-64 Apaches... which hovered above it and pounded it with hellfire missiles, hydra rockets and 30mm cannons.

But assuming that didn't happen.

The French came clean about the whole thing, which they clearly knew about the whole time and did nothing to prevent, and everyone was very upset with them.

But assuming that didn't happen.

Godzilla was in fact, a slumbering prehistoric monster like he was supposed to be. The bomb detonated... and killed him, as prehistoric monsters are just as vulnerable to radiation, high temperatures, and blast effects as iguanas and worms.

The worm was there as well for some reason despite that being a massive time paradox. Accordingly, it was erased from history.

But assuming that didn't happen.

Godzilla awoke, and then realised all his friends were dead and was sad. Japan built a giant dancing robot for him and everyone was happy!

But assuming tha... Oh, screw this, realism my ass.