A voiced version of the given chapter will be available from the given time, as well as challenge versions of the related missions. Press チャレンジ クエスト on the bottom left on the main story selection screen to access these challenge missions.
Note that a "Replay Story" button is also available, activate this by setting 既読ストーリー再生 to ON. This button is found on the battle select screen, next to the menu button.