A saved-up story with 〜ておく

Grammar Explanation: Doing in advance with 〜ておく

A sleep-over at Chino’s place turns into a late-night scary story-telling session.

千夜(ちや):: 「とっておきの話(はなし)があるの」; “I have a special story in reserve.”; Literal: “I have a story that has been taken and put aside.”

とっておき acts as an expression (取(と)る + おく), with a meaning along the lines of “to set aside; to keep in reserve; to hold on to”.