During the production of the 2009 anime series Bakemonogatari, Supercell was asked to compose what would later become the ending theme song "Kimi no Shiranai Monogatari".

[6] "Kimi no Shiranai Monogatari" is a J-pop song with instrumentation from electric and bass guitars, drums and piano.

According to a book of sheet music published by Yamaha Corporation, it is set in common time, and moves at a quick tempo of 165 beats per minute in the A major key throughout the song.

[10] The introduction starts with only piano accompanying Yanagi's vocals, and uses a bridge with added guitars and drums to transition into the first verse.

The cover artwork features an illustration by Shirow Miwa with the Summer Triangle depicted in the starry sky.

[2] "Kimi no Shiranai Monogatari" was released in limited and regular editions on August 12, 2009 as a CD by Sony Music in Japan.

[18] In January 2010, "Kimi no Shiranai Monogatari" was certified gold by the Recording Industry Association of Japan (RIAJ) for 100,000 full-track ringtone digital music downloads (Chaku Uta Full),[19] and was again awarded a Gold Disc by the RIAJ in February 2010 for having exceeded 100,000 copies shipped in a single year.

The video, directed by Hirohisa Sasaki, does not feature any members of Supercell nor the singer Nagi Yanagi.

[21] Instead, the video prominently follows the actors Marika Fukunaga, Shuto Tanabe and Naho Kitade, who play members of a high school astronomy club with five others.

The video ends with a flashback showing Fukunaga, Tanabe and Kitade happily gazing at the Summer Triangle at Inubōsaki lighthouse.