The story is in relation to ZUN’s (author) and Hirasaka Makoto’s (illustrator) manga, 「Touhou Sangetsusei ~ Strange and Bright Nature Deity」. Apparently the ice fairy Cirno has declared a war against the Three Mischievous Fairies: Sunny Milk, Luna Child, and Star Sapphire.
In the game you play as Cirno and enjoy freezing bullets that you’re suppose to dodge. Also interesting to note is that TH12.8 will feature artwork from Hirasaka Makoto. A full version of the game will be released at Comiket 78 — August 13th-15th.