This ova seems to take place after the first season, at least Ep2 and Ep3 do.
It tells from the perspective of the EU, which is at war with Britain. In the further episodes, some fragments are mentioned, to the missing time gap of Lelouch.
Otherwise, new characters are introduced here that have no further relevance to the actual Code Geass plot and only their story is told.
So far it's hard to judge the characters, but this Ova lacks charm. The CGI of the robots is pretty mediocre and the characters so far are only stereotypes.
Conclusion
Possibly recommendable for diehard fans of Code Geass, but the clear drop in quality seemed anything but convincing after the first ova.
