pre-noun adjectival
a certain ...; some ...
Pre-noun adjectival (rentaishi) used to refer to an unspecified but particular person, place, or thing, often in narrative contexts.
とある町で小さな本屋を見つけた。
I found a small bookstore in a certain town.
とある理由で彼は来られなかった。
He couldn't come for a certain reason.
A strange incident occurred in a certain town.
とある人が彼にこの本を渡した。
A certain person handed him this book.
One day, she suddenly disappeared.