Promise (Yakusoku - 約束) is the 20th episode of THE iDOLM@STER anime series. It first aired on November 18, 2011.
Chihaya's tragic past is revealed in a magazine no thanks to Kuroi which reveals that Chihaya had a younger brother named Yū who died in car accident when she was eight but the magazine slanders her, suggesting the shocked Chihaya who was there did nothing and was heartless and was also responsible for her parents divorce. Chihaya reveals to Haruka that she became an idol to sing in memory of her late brother who loved her singing but because of the revelation of her past has traumatize her that she is unable to sing, Chihaya wants to leave 765 Pro studios. As the days past, Chihaya refuses to leave her apartment, much to everyone's worries. Haruka tries her best to help Chihaya but Chihaya scolds her for being a busybody. After receiving Yū's sketchbook by Chihaya's mother to give to Chihaya herself, Haruka asks the Producer if she's just a bother for trying to help everyone but he assures her she isn't as it was her enthusiasm that help him when he was down. After reading Yū's sketchbook, Haruka and Miki have an idea to help Chihaya. Everyone at 765 Pro wrote and sent a song for Chihaya about their feelings to her which they hope she would sing it at the 765 Pro Autumn concert and Yū's sketchbook which Haruka reveals that what made her brother happy wasn't her singing but seeing Chihaya smile. On the day of the Autumn concert, just when it looks like Chihaya won't make it for her part, Chihaya finally arrives much to everyone's relief. As the music to her song begins, Chihaya is still unable to sing until Haruka and the rest of the idols sing the song for her. With their help and seeing her past self and little brother, Chihaya is finally able to overcome her trauma and finish the song herself. As the crowd applause her, everyone cries with Chihaya in celebration.
Promise (Yakusoku) by Chihaya Kisaragi (Asami Imai), Haruka Amami (Eriko Nakamura), Iori Minase (Rie Kugimiya), Azusa Miura, Ami and Mami Futami (Asami Shimoda), Yukiho Hagiwara, Hibiki Ganaha, Takane Shijou (Yumi Hara) and Yayoi Takasuki (Mayako Nigo).