Whoa, the episode shows some of Kiritsugu's past when he was a child. It was pretty much dark-ish near the end though. We also get to learn about his farther and find out about his big secret along with our protagonist's best friend. It was kinda nice that the series introduced two organizations and showed a bit more info on what they do. Other than that, not much to say because this a filler on the past events of the series. But, I do wonder when the creators might show more of those past events. So overall, this is a pretty neat filler.
Conclusion: A filler episode about Kiritsugu's past.