This OVA tells us one short story about Lafiel's Parents, set way before the events of the main series. However, because of a small detail, it might be interesting to watch this only after you watched Seikai no Monshou, but be sure to watch it BEFORE Seikai no Senki II

Pacing

With the same interesting environment as the whole seikai series, this short story is not in the books, but was actually published in a Japanese magazine. This is quite an interesting flashback about not only Lafiel's parents, but also about the Abh society and history.

If you are a Seikai fan, this is quite a golden eastern egg - don't miss it.

The best sequence to this series is: Seikai no Monshou - Seikai no Senki - Seikai no Danshou (this OVA) - Seikai no Senki II - Seikai no Senki III

Conclusion

Seikai no Danshou adds a lot into Lafiel's past history and the circumstances of her birth, and what is more important, it adds to the plot the statement "Watashi no kawai denka" (My cute princess), which will have a huge emotional impact at the ending of Seikai no Senki II ;)