Strike the Blood: Kieta Seisou-hen take place after III and gives us a mystery of how Kojou and Yukina somehow ended up in a futon in their school sleeping together, with Yukina's Sekkarou gone and her in a PE Uniform, a bloody bat and none of them have any recollection until Kojou and Yukina finds Nagisa and latter on Sayaka and La Foila to get them to remember...."what the hell happened?". There is a small sub plotline in between this with Yaze, Kiriha, Shio and Yuiri investigates Vatler's sunken ship and comes across trouble.
LA immediately thought this OVA plot was taken from
"Dude Where's My Car", Or "The Hangover" as the plot structure is as such, what with Kojou, Yukina and us the audience retracing events of this mystery to see how things became to be and how Kojou and Yukina ended up like they did and yes because this IS Strike the Blood were talking about and this IS an OVA, fanservice abound, from the harem being in fanservicey outfits to some nudity, mainly Yukina and Sayaka (Praise the Vampire Gods).
The "mystery" goes in a more fanservicey direction than a comedic direction though there is comedy involved in the quote unquote "comedic" situations Kojou gets himself into to.
The animation done once again by Connect is pretty much the same as the third season's quality of animation, so not much to say about that. The voice cast is back and really not much to say about this either...ehhh maybe Risa Taneda, Asami Seto and Ikumi Hayama had a few good lines here and there.
Really for an one shot isolated episode, Strike the Blood: Kieta Seisou-hen is a nice reprieve from the madness of the third season with some harem hijinks under a "Dude Where's My Lance?" format with fanservice added in because of course and LA liked it all things considered for a one episode arc, the other reason is that this incident is a leadup to the new villains of Strike Blood IV that's to come...