Natsu no Kagerou (なつのかげろう, Heat Haze Summer) was a collaborative game being developed by Ryukishi07 and Twilight Frontier, who had previously collaborated on the 3D fighting game Higurashi Daybreak. The story would follow a boy named Kousuke who reunites with his childhood friends, Sae, Wakana, and Youko, as he returns to his remote hometown of Miwata Village. A demo was released in 2011, however the project has since been abandoned. Ryukishi07 adapted much of the story for the manga Hotarubi no Tomoru Koro ni.
Natsu no Kagerou was advertised as an adventure horror open-world game. The player assumes the role of Kousuke and explores the remote Miwata Village over the course of four days. Gameplay is divided into day sections and night sections. During the day, the player is given several minigames based around improving Kousuke's relationship with the three heroines. At night, the player must survive encounters with malevolent spirits called mōryō.
The game was first announced on Twilight Frontier's official website on April 1, 2010. According to Ryukishi07's production diary post on April 2, 2010, he was first contacted about making a game similar to Siren after the completion of Higurashi Daybreak and subsequently began work on it. For the scenario of the game, Ryukish07 contacted G. O. from the doujin company Blank Note, known for the visual novel Himawari -The Sunflower-.