The first part in a short film trilogy, following a TV series. It's a pretty basic anime full of familiar tropes, though sporting a rather peculiar setting. That means it might take some time to get the hang of the characters and their particular function and skills, but if you've seen some anime series before, it shouldn't take you too long to catch up.
In an alternate Britain, the country is divided into two different parts, separated by a wall (think Germany 50 years ago). While there is no outright war between the two nations, they love to spy on each other, and spy activity is ramping up. Dove, a collective of young female spies, is assigned a case where a double agent is suspected of having infiltrated the royal entourage.
The art style is a bit too slick for my taste, but the animation is decent and the film looks pretty colorful and detailed. Once you get used to the unique setup the rest of it is pretty by the numbers, even so, it never gets boring or too generic. Not the greatest anime out there, but I'll be certain to catch the remaining films, as it's far from the worst too.