The events that transpired from the start to the end of the Rain World universe are complicated and entangled. The slugcat's journey across the world is also riddled with clues and subtle messages of the past. Far more than just finding a home and a return to its family, its adventure is also a search for truth.

A number of pieces of evidence and excerpts acquired over the course of the game's storyline offer a logical and well-supported explanation in regards to the cause of the world's current state of decay and disrepair.

The Ancient Civilization and Void Fluid
Eons ago, an ancient civilization inhabited the planet. Intelligent and capable, they ruled supreme over the realm. However, even such a developed society could not escape the grasp of the Great Cycle that impacted all living creatures. Birth, death and rebirth were connected to each other like a ring, and try as they might, none could truly die. Death would simply result in a new awakening—a restart, a repeat. Stuck within a deathless loop of agonizing longevity, it had long been this civilization's goal to break free of this cycle—a process known as transcendence.

Even far before the storyline of Rain World began, methods and spiritual instructions were devised by beings such as monks to rid oneself of natural urges. They were practiced in an effort to escape the Great Cycle. However, there is no indication that these instructions were able to achieve any success.

The situation changed drastically, however, with the Void Fluid Revolution.
At some point in their history, the ancient civilization discovered what later came to be known as Void Sea far below the surface in the Depths. The Void Sea was comprised entirely of Void Fluid. While it was difficult to harvest the fluid due to its nature of dissolving anything that comes into contact, devices known as Void Drills were invented to gather up small amounts of Void Fluid embedded in rocks slightly above the Void Sea, where its concentration is lower and wouldn't pose a risk to the equipment. The impurities such as rock and sediments were then filtered out to obtain a sample of the pure liquid.

Soon after this, the fluid was found to be an outstanding source of energy. Armed with the potential of the Void Fluid, technological advancement boomed, and the ancient civilization reached the height of its power.

After further studies, it was found that the Void Fluid does not just dissolve objects, it also removes them from the physical realm. Attempts were made to dig even deeper into the Depths to find higher concentrations of Void Fluid as well as uncover more information about it, but the ancients' equipment simply broke or dissolved. As later described, no one returned if they ventured too deep into the Depths. However, this property of the Void Fluid coincided with what the ancients thought to be the solution to escaping the Great Cycle. The civilization believed that the ability of the Void Fluid to remove one from the physical world was the transcendence they had longed for. Since any that came into contact with Void Fluid did not return, it was inferred that they must have broken free of the cycle. However, a risk was associated with taking the plunge into Void Sea to escape the world. Not even Void Fluid was enough to remove those with sufficiently large egos from the physical realm. This resulted in some "horror stories" in regards to semi-transcended abominations. Several ancients that did not entirely transcend are likely to be the six "blue-golden, glowing, squid-like creatures" scattered in the world. These creatures claim to know how to transcend, but express a sense of regret and sorrow at being trapped between the mortal realm and the transcended realm. The names of the six ancients are:

Nineteen Spades, Endless Reflection
Six Grains of Gravel, Mountains Abound
Four Needles Under Plentiful Leaves
A Bell, Eighteen Amber Beads
Two Sprouts, Twelve Brackets
Droplets Upon Five Large Droplets
It would seem that, in order to transcend, one would have to abandon all attachments and desires, entering an "effortless" state. The ancients once again turned to archaic methods to rid of one's natural urges and ego before deciding to use Void Fluid in order to avoid failure in transcendence. However, this seemed not to be an entirely perfect solution, which would explain why iterators were constructed (see below).

Creating and Using the Iterators
It was around this time that the height of the civilization's power had passed and it fell into an age of decline. The cause for this decay has not yet been revealed. Believing there to be an alternative method to transcend without the risks associated with Void Fluid, the ancient civilization used its technological prowess to create and construct the iterators. Iterators was the name given to supercomputers or artificial intelligences that were constructed to solve the 'Great Problem'—to find the method of breaking free of the Great Cycle. Each was of a colossal size and held incredible power, and it was their purpose to ponder and uncover a method to transcend without the use of Void Fluid.

Seven iterators have been named but there are up to twelve total (including the named seven) which have been seen in-game. The seven mentioned are:

Five Pebbles (Erratic Pulse)
Looks to the Moon ("Big Sister Moon" to her friends)
Sliver of Straw
Seven Red Suns
Chasing Wind
Unparalleled Innocence
No Significant Harassment (NSH)
The Iterators were a combination of organic and synthetic components, relying on specially grown microorganisms as peripheral processing devices: their natural metabolism and reproduction could perform very complex calculations beyond the capabilities of electronics physically. In order to keep their electronic components cool and their organic components clean, iterators required absolutely gargantuan amounts of water. The iterators stripped their nearby ecosystems of water and pumped water vapor back into the atmosphere, where it would condense into rain and be used again. Due to this process and the sheer number of iterators built, the equilibrium of the world's ecosystems was severely disrupted or even destroyed, creating a dense layer of fog and clouds rife with torrential downpours and periodic flooding. Ultimately, this effect came to be known as the Rain. The ancient civilization compensated for this by moving away from living on the surface, and constructed immense, dense cities directly on top of the surface of the iterators' "cans"—massive, square shaped buildings which protruded like islands far above the cloud layer. In addition to computing the solution to the Great Problem of transcendence, the iterators were tasked with providing regular shipments of food, water, equipment and Void Fluid for energy to these colonies.

Fall of the Ancients and Pebbles' Madness
Unfortunately, the ancient civilization disappeared and collapsed before the iterators computed the solution to the Great Problem due to an as-of-yet unknown reason, possibly because the iterators could no longer satisfy the needs of the populace, or perhaps a massive transcendence movement of the entire population, leaving the surface cities dark and abandoned if largely intact. However, the iterators were left turned on, their final command being to continue working towards a method to transcend without using Void Fluid and guide lesser creatures on the path to the realm beyond should the solution ever be discovered. Over the next indeterminate amount of cycles up until the start of the game, the iterators did not achieve any major breakthroughs. Only Sliver of Straw claimed to have uncovered the solution, but she died almost simultaneously as the confirmation of success was broadcasted to the other iterators, causing the methods in which she attained this positive result to be lost.

Unable to find the solution after Sliver of Straw's death and growing increasingly frustrated, another iterator—Five Pebbles—became convinced there was no solution to the great problem, there was no reason to keep trying to solve it, and that death was the only way out. It is likely he showed these beliefs off in iterator chat logs under the pseudonym EP. The ancients had anticipated this kind of thinking, however, and put taboos preventing self destruction within the iterator's genetics. One (and perhaps the only) way to circumvent these taboos was a very dangerous and repetitive shuffling of genetics that required a heightened water intake. The goal was to cause cells to mutate until the desired genetics were obtained: ones that would overwrite the encoded taboos. It is very likely Five Pebbles used this method to attempt overwriting his genetics. At some point Five Pebbles began drawing several times the usual amount of water, tapping into the neighbouring iterator, Looks to the Moon's water source. Attempts by Moon to persuade Pebbles that water shortages would be fatal to her failed and she had to leverage her seniority in an attempt to make him stop, to which Pebbles responded that this plea has "ruined everything". Upon being interrupted, Five Pebbles lost focus of his genetic shuffling, and it is likely it became what is known as the Rot (or Daddy Long Legs). Five Pebbles attempted to rid his systems of the rot, doing things such as flushing it out into Garbage Wastes, but it has proven resilient and still resides within him, slowly consuming more and more, day by day. It is unknown if Five Pebbles still uses extra water during the time that the three playable slugcats take place in. Due to many cycles lacking water, Looks to the Moon's processes eventually seized, her facility compromised, and her primary cognitive unit descended into an amnesic, near-death state. The fate of Seven Red Suns, Chasing Wind, and Unparalleled Innocence is unknown, but No Significant Harassment is active and well at the start of The Hunter's story.

The Timeline of Hunter, Survivor, and Monk
Hunter is sent by No Significant Harassment with a pearl and a slag-reset neuron to help Moon in her state of decay. The Survivor's story is based on the scenario that the Hunter delivered the neuron and revived Moon, with the rest of its choices unknown. In the Hunter's transcendence scene, the Hunter is seen with a strange purple goo coming out of it, before being seen in the arms of an iterator-like figure.

One interpretation of this cutscene is that the Hunter was injected with some of NSH's microbes, and then sent by NSH to transcend under the cover of helping Moon. The goo seen coming out of the Hunter is believed to be these microbes. This also explains the Hunter's sickness- like in the Iterators, slag would build up off these microbes being worked, and would thus lead to organ failure in the organism. By doing this, NSH was able to effectively break the self-destruction taboo, enabling them to transcend with the Hunter.

A second interpretation is that the goo is the Hunter's sickness (in this case presumed to be either cancer or some other terminal illness rather than slag buildup) being removed from its body upon transcending. The iterator-figure is, in this scenario, theorized to be Sliver of Straw, the only iterator to ever transcend. She then carries the Hunter into the afterlife, allowing it to rest.

Some time after these events, Survivor is separated from its family by heavy rainfall, and is eventually guided by one of Moon's Overseers. The Overseer leads the Survivor towards Moon with images of its family, and encourages the Survivor to bring additional neurons to repair her memory. After visiting Moon, the Survivor travels to Five Pebbles, achieves the Mark, and transcends. However, the player can choose to not transcend and/or steal neurons from Five Pebbles and give them to Moon. The player can also skip visiting Pebbles and Moon entirely, and instead pursue the Echoes to gain maximum karma and transcend that way. The Survivor, upon transcending, is taken to the Void where many other transcended slugcats are shown.

The Monk is seemingly a younger sibling to the Survivor, jumping after its older sibling during the beginning cutscene. The Monk's story seems to take place a significant time after the Survivor's story. This is shown in-game by the fact that the world's data pearls have faded and no longer contain information. The Monk is left with the same options as the Survivor in terms of meeting the iterators and transcending. As of 1.7, it is revealed in the Monk's ascension cutscene that it finds the Survivor in the Void.

The ending cutscenes could also be interpreted as the slugcats' wishes manifesting as final dreams as they lose consciousness in the void fluid: the Hunter dreaming of being back with NSH, the Survivor finding its family and the Monk being reunited with its sibling.

As violent as The Rain is, an ecosystem has developed under the bullet-like droplets. It is in this decaying and crumbling world of mysteries the slugcat's adventure commences.