Flowerfell was created by Siviosanei, where every time Frisk dies, the timeline resets. And when they return to life, a yellow ranunculus or "buttercup" grows on their body. The flowers could not be removed as it hurts Frisk. Despite the cruel nature of the characters in the Underfell AU, Frisk remains pacifist. Only Sans is aware that the timeline is being reset. After several times he has killed Frisk, they still showed him kindness. This eventually leads Sans to feel compassionate towards the human, realizing that the "to kill or be killed" mantra of Underfell isn't necessarily correct. He accompanies Frisk to the barrier.
The story begins after Frisk dies for the first time in Toriel's hands, and wakes up with a flower on their head. Flowey is the first to realize that a reset occurs everytime the human dies, however he cannot explain the flowers. Despite constantly being attacked and having died repeatedly, Frisk refuses to fight Toriel. Eventually, Toriel decides to let them out of the Ruins.
The first time Frisk meets Sans, he kills them after their first handshake. On some resets, Sans decides to kill Frisk himself. And on others, he betrays them and leads them to his brother Papyrus. However, Frisk returns and continues to treat Sans with kindness. This intrigues and confuses Sans, and he eventually invites Frisk to eat at Grillby's, where they order fries. Sans takes the mustard, and Frisk takes the ketchup. In each reset where they eat at Grillby's, Frisk would thank Grillby for serving them their order. Later on in the story, Sans would reveal that he realized Frisk is aware of the resetting timelines, because Frisk knew to hand him the bottle of mustard.
Sans would ask why they act nice towards others, such as how they would often say "thank you", which is uncommon. Frisk tells him that they can be rude, but they don't have to, and don't want to. They would tell him to "Always be kind." In several resets, Sans also asks where Frisk intends to go out. To which, they would answer "out". On one reset where they were asked this question, Frisk invites Sans to join him to go back to the surface. Eventually, Sans would contemplate on the possibility of people acting good (as is opposite to the nature of characters in Underfell AU). Sans would argue with Papyrus, attempting to convince him that the human may not be worthy of killing. Unable to convince Papyrus on befriending the human, Sans instead tells him that he is taking them to Asgore. In truth, Sans is accompanying them to the barrier and help them return to the surface.
When encountering Undyne, Frisk begs Sans not to hurt her. It takes them several attempts until they manage to survive the battle long enough for Undyne to be tired out, and Sans teleports them out of there. They also encounter Alphys, who calls Flowey by the name "Asriel", and calls him a traitor.
As Sans protects Frisk against a other monsters including Mettaton, and Muffet, Sans would eventually grow frustrated and kill Muffet. Frisk does not force a reload, however Mettaton eventually kills them. After this reset, Sans would avoid killing Muffet.
At this point, Frisk has grown blind due to the flowers growing over their eyes. Their limbs have been consumed by the flowers. They could no longer walk, and have a difficult time talking. They would shakily use sign language to communicate with Sans.
When encountering Asgore, Sans is able to weaken Asgore only enough for him to yield. Asgore coaxes Sans to finish him off, but Sans refuses. He tells Asgore, "always be kind," the same way Frisk had told him before. Asgore thinks this is foolish, reiterating that in this world, it's "kill or be killed".
When they reach the barrier, Flowey suggests they could take one of the six souls, cross the barrier, and bring back one more. After this they could break the barrier for good and free everybody. Frisk refuses, and urges them to use their soul instead as the seventh soul. This devastates Sans, however Frisk told him that they could not continue living on with the way their body had been consumed by flowers.
After Frisk has destroyed the barrier, Asgore acknowledges the human's sacrifice for the good of monsterkind. In turn, he called off the plan to destroy humanity. They build a community, and Sans now lives with Flowey. Often, Sans would visit the Ruins where they had buried Frisk. He would wait for them to come back, just as how the timeline used to reset every time Frisk died.