This AU takes place in a post-apocalyptic desert-like world where civilizations fight over water.
In a big city that has taken claim over the largest sources of fresh water, a scientist is ordered to create a weapon to conquer more water-settlements so the city can control them instead. The scientist does what is asked, but as his creations, called Gaster Blasters, grow and start to show self-awareness and intelligence, he refuses to let them be used for military gain. When he tries to go into hiding he is promptly shot for ‘treason’.
When the government tries to take control of the two creations, however, they quickly bump into a huge problem: The scientist could not speak, and he wrote and signed in his own made-up language. So they can’t use his notes to create more weapons (most of the notes were burned beforehand anyway), and the two existing creatures were trained in and would only answer to the doctor’s sign language. They do end up finding a use for the two brother creations though, as a method of execution.
The two creatures, especially the younger one, do have a strong instinct to kill ‘corrupted’ people (murderers, rapists, high degree abusers, etc.), and they find out the older of the two will do anything to keep his brother from harm. They turn it into some kind of game show (a la Hunger Games) where they send criminals (both major and minor, they pull straws to pick them) into a jungle enclosure where they have to run and try to survive, with the two creations on their heels, until all of them are dead. Usually, it takes just a day, once a month, with Papyrus impulsively (and not remembering afterward) going after the heavier criminals while Sans goes after the more ‘innocent’ people to spare his brother from doing so.
Frisk is a mute orphan living in the streets. They know the universal sign language that is used by the people, but ever since birth, they also know another kind of signing. It just came to them ever since birth, no one else understands it, and they sometimes mistakenly start to sign in it, especially when they’re nervous.
One day Frisk get’s caught for trying to steal some food and their number is drawn to be executed. They and about 9 other criminals are released into the arena, where they immediately scatter to get out of there before the two monsters are released. Frisk hides and it actually takes Sans quite a while to find them. When he does there is a short chase before he corners them. In their panic, Frisk starts to plead for their life in their own sign language and Sans actually stops, confused. To keep up the show he pretends to eat Frisk whole, while actually keeping them safe inside his jaws as he smuggles them into their stables.
Once Sans is sure no one will see the kid, he spits them out and Papyrus and he instantly starts to feel a kinship with the child. In turn, Frisk’s brain starts to fall in tune with the two brothers. Their movements, their sounds they make, even the way the brothers glance at something/someone, it all makes sense to them. Eventually Frisk learns the codes that unlock the stable doors and shock-collars of their friends and they manage to escape into the desert outside the city.
After running for hours (and Frisk suffering under the burning sun) they decide to stop near a tiny settlement. It is built in the Ruins of a village that died out after it’s well dried up, but the new inhabitants found a way to dig the old well deeper and be more thrifty with the new water source.
The trio decides for Sans and Papyrus to stay hidden in the dunes while Frisk goes to see if they can get their hands on food and water. Frisk get caught (again) trying to steal some apples, but this time by an elderly lady that gently scolds them instead. When Frisk admits that they don’t have a home or family to return to the lady introduces herself as Toriel and invites Frisk to stay with her. Frisk refuses and tells Toriel that they have to return to their ‘brothers’. After being offered and once again refusing for them and their ‘brothers’ to stay with her, she buys a bag of apples and a waterskin for all of them. She also gives Frisk 3 large pieces of fabric they can build a shelter from against the burning sun and freezing night: A long strip of red cloth, a blue, aras style rug with fluffy white tassels at the ends, and a blue and purple striped shawl.
They put the red fabric around Papyrus’ neck (so they don’t lose each other in a sandstorm), the rug around Sans’ rib cage (to protect and hide his fragile soul) and they themself wrap the shawl around their head (against the sun).
Before they can leave though, another tribe appears with the intent to take over the tiny oasis. Within the community of the Ruins, there are only a few person that is both trained and able enough to fight off the intruders, lead by captain Undyne.
Undyne’s forces are too small and she desperately fights off the nomads to try and save her village, and then it turns into a struggle so her wounded soldiers can be brought back to safety. Witnessing the clash between the honest desire to protect her loved ones and simple greed, Papyrus’ instincts kick in and he rushes into battle. Sans and Frisk don’t have much choice to go after him.
The opposing forces rapidly grow frightened of these two strange, powerful creatures and their ‘rider’, and they quickly decide to retreat. They take Undyne back to the village, where they are met by its equally terrified inhabitants. Toriel, however, recognizes the articles of clothing the trio is wearing. She introduces herself as the leader of the settlement and tells Frisk that, even though she now realizes why they refused her offer before, all three of them are more than welcome to stay with her. They accept this time.
Then Undyne and Papyrus quickly grow to be friends and battle-buddies, though she’s kinda pleasantly surprised that this ferocious war machine she saw before, is actually a giant, friendly skeleton puppy. Alphys, who is now the village’s scientist, used to work for Gaster, she was lucky to escape since Gaster told her to get out of the city before he could start packing. Thus she actually knows the brothers inside and out and helps to train them and bring out their full potential.
Together they’ll be going against the misguided Asgore (unstable after the death of his children during an accident with the escape of a few criminals) and his wicked and power-hungry advisor, Flowey.