Nazo no Kanojo X: Nazo no Natsu Matsuri
English title: Mysterious Girlfriend X: A Mysterious Summer Festival
Japanese title: 謎の彼女X 謎の夏祭り
Type: OVA
Source: Manga
Episodes: 1
Status: Finished Airing
Aired: Aug 23, 2012
Duration: 26 min
Rating: PG-13 - Teens 13 or older
Studios: Hoods Entertainment
Genres: Mystery, Romance, Ecchi
Demographics: Seinen

OVA released with the 9th volume of the manga. 

The day when Tsubaki goes to the summer festival with Ueno, Oka, and Urabe.