The anime industry loves compilation movies. They're cheap and easy. You've already done the real work in making the TV series. If it was successful, though, there's some old rope waiting to be sold. Nip into the editing suite for an afternoon and, bingo, you've made something to show in cinemas!
This film is both a compilation movie and a sequel. A year after the events of the TV series, the Super Peace Busters look back on their time with Menma and... well, don't really do very much, to be honest. But it's still charming. That's the framing story. Most of the film is TV series flashbacks, presented out of order and roughly arranged by character.
I thoroughly enjoyed this film. Anohana still has emotional power. My eyes prickled. However the best way to experience this story is clearly the original TV series, which is extraordinary. Eleven 23-minute episodes, in order. That's four hours of material. This film still has a lot of force, but it's not the same. Would it make sense and have weight for someone who's never seen the TV series? I suspect it probably would, actually, since Anohana isn't a plot-driven story and I think anyone should be able to work it out. The main obstacle for an audience is simply that this film is split between three time zones (characters are 10, 16 and 17), so it can sometimes take you a moment to work out which version of the older-teen characters you're watching now.
Of the film's additions...
The framing story doesn't add much. I'd scream and howl if you tried to take it away from me, since I love all things Anohana, but it's not really trying to move the story forward. Is there any more, in fact? The dead girl's still dead. The Super Peace Busters are at peace. There are still some one-sided feelings that may or may not end up being reciprocated, but to be honest ep.11 of the main series gave us stronger evidence of what's going to happen there.
The new flashback material, though, is important. It fleshes out Menma. We'd never really seen before why she was so focused on the gang and its members. This is the first time we've dug into her inner life to this degree.
There's not a lot to say about this one, really. It's Anohana. Everything I said about the parent series is true here too, because that's most of what this film is. Re-edited and re-ordered, but basically the same story with twiddles. It's still a hell of a story, though, and I like those twiddles.