It happened in the past…
Iroha would go to the hospital daily and play with Ui, Touka and Nemu.
The girls loved Iroha and looked forward to her visits.
One day, the three girls create an Uwasa similar to an urban legend while playing.
That way, if they died, the Uwasa they created would live on and remind Iroha of them.
Iroha also created the “Uwasa of Eternal Sakura”, with the wish that the three girls will be discharged from the hospital without illness.
And one day... Kyubey appears before Iroha.