Saikoroshi-hen is an epilogue of Matsuribayashi-hen which takes place two months later, in August 1983. Rika is run over by a truck and wakes up in a totally different world, where none of the tragic events of the main series ever happened: Keiichi is not in Hinamizawa, Rena's parents did not divorce, the dam project was resolved smoothly, Satoshi did not run away, and neither Satoko nor Rika's parents died. However the club never formed, meaning Rika has no friends, she gets bullied by Satoko, Hanyū is absent, Rika is not revered as the reincarnation of Oyashiro and the village will soon be submerged underwater. Rika has to choose between staying in that world or killing her own mother, which will enable her to leave that sinless world.

(Source: Wikipedia)