I quite enjoyed it. I was expecting to be all at sea, mind you. I hadn't seen the original Love Live! in ages, while even Love Live! Sunshine!! (the sequel) aired in 2016-17. What's more, these idol anime tend to star lots and lots of girls, so you'd have to be a mastermind to remember all of them if it's been a while.
Fortunately, though, you don't need to remember the characters to watch this. I'm impressed by how immediately I recognised Nico Yazawa, though. She was always the loudest, funniest character, but even so she'd clearly rooted in my memory more firmly than you'd expect. (Yes, I laughed. "It hurts to admit it, but she's cuter than me!")
It's a story about dreams. Maki Nishino runs down darkened catacombs, pursued by a spotlight. She enters a cathedral, then gets trapped inside a cage as hooded, sinister figures surround her.
This has to be a dream, but you'll be begging for it not to be. It is. She's in the classroom, telling Hanayo Koizumi about it (and scaring her). Later, though, the dreams return. There's a mysterious child, a silent key on Maki's piano and a forgotten textbook that leads to more nightmare chasing through impossible darkness after a shadow. Day becomes night. A dream sequence segues into apparent reality, but with the girls discussing the possibility that they're still inside a dream.
There are two Makis. She admits that she's always loved everyone and you wonder where this will go next...
Answer: a music video. The girls perform one of their stage numbers. This OVA has ten minutes of story, then five minutes of idol singing and dancing. The end. Even the song's interesting, though, because it's at odds with both the end of Season 2 and the movie. Love Live! ended, twice, with the girls disbanding. They made a big deal of it. The song, though, is Music S.T.A.R.T!! and is full of lines like:
"Hey, the party doesn't end"
"The spiral of light won't disappear, never, it won't disappear"
"So this place is our never ending stage"
"(We won't stop, everyone) La la la this is the best feeling!!"
Apparently the girls' training outfits suggest that this OVA is set between ep.3 and ep.4 of season 2. Personally, though, I think it belongs in release/broadcast order, i.e. between seasons 1 and 2.
It's pretty bonkers. Is it all a dream? If so, whose? I'd have loved to see them explore all this further, but I'm also amused by the way they just guillotine the madness in order to sing and dance. Eli Ayase says "khorosho" in a situation where the word makes no bloody sense, but that's not new. (It's Russian for "good" and she's a quarter-Russian, as per the backwards-R she used to have on her practice clothes... yet neither of her names begins with "ya". This franchise seems to be trying to annoy Russian-speakers.)
I enjoyed this. It's a laugh.