Sometime back, I wrote a largely enthusiastic review of Vivid Strike!, which I consider the best series in the maddeningly inconsistent Magical Girl Lyrical Nanoha franchise.
As far as I have been able to discern, the series is only legally available, at least in the U.S., on Amazon Prime—and I recently saw that the series is slated to disappear soon. At the time of writing, Amazon has posted a notice that it disappears in nine days. Assuming I can count, that means it will be gone on October 1st.
The rest of the Nanoha shows have previously disappeared from Amazon and are, as far as I know, not available anywhere else to the English-speaking audience.
Despite its problems, I do think ViVid Strike! is one of the best encapsulations of the magical girl warrior concept. If you’ve not seen it, you might consider watching it before it disappears. It doesn’t require knowledge of the other titles in the franchise.
