If you haven't watched or read the series or haven't even got to the end of either adaptation, back up and walk away. This is an end-film, the last hoorah to the anime. I personally haven't watched much of the anime but I have read the manga so it's safe for me at least. And content-wise, it provides the biggest spoiler I can think of, and generally revolves around that and having a party in Godai's room because when don't they have a party? Drunken party-goers abound and the booze flows like water, just like the stream of characters that randomly walk in.
And by random I mean nearly every damn person that was in the series and others even get mentions. From the suspected Kyoko's dad and Tennis Coach Shun with his trademark glinting teeth to the obscures; the puppet show club members and Mr. Ichinose. Yes, that guy. You don't remember him but he shows up, I'm sure of it. It feels pointless that they do, in a way. Kinda like they're trying to get all sorts of cameos in because it's the end of the series anime-wise.
It does tie up some loose ends and helps others come to a conclusion but it overall feels unneeded as I'm sure the anime ended just fine and this is more of an after-party. It's hardly a perfect conclusion and a lot of it just feels pointless but it felt good to return. I loved the series so I'm giving it an extra point or two but unless you're a major fan of this series, it's best to skip it.