The story is set before the main story's "Battle in Nagoya" arc.
The Shinoa Squad receives another mission from Guren; to rescue a human girl named Riko from a mysterious vampire, Shahar, and bring her back to her family. However, Ferid and Crowley also appear when they find Shahar. Can Yuuichirou save the girl?
This is a very good ova. In most animes the ova will involve the characters just messing around at home or at the beach or at the hot springs, etc. But this one actually has a story, and a fleshed out one at that. The studio also does a good job at fitting the ova in-between the two seasons, even though they made this after the second season concluded. The only drawback I found, was a few little inconsistencies with the characters' personalities, but that can almost be expected in ovas. The sound and music was just as good as the main story, so you
won't be too disappointed there. If you want to see more of Owari no Seraph after finishing the main story, this is for you, as it adds more action and a good mini-story. All in all, it deserves a 9.5 or higher.