The plot is very interesting. It shows us that there is something more in the titans. What the heck happened? After watching this I got a feeling that the titans actually don't wanna eat humans but they are somehow forced into it. Heh such an interesting topic. "The spirit truly is ready, but the flesh is weak." Mark 14:38
Anyway, basically you return some time before the ending of the first anime season and watch some well-known characters (Levi and his squad, Hanji, Ervin..) discovering another fact about the titans. The reason why Ervin allowed Hanji to capture titans alive. Don't wanna spoil but we meet here a new character who died a year ago but brought to light an incredible information about titans in a small notebook.
So this OVA was really interesting piece of puzzle to the mystery behind titans. It was made in the same style as the anime so I take it just as another episode of the first season.
Oh and by the way hahaha Oluo from Levi's squad bit his tongue again :D
