Plot
The season is summer, but for some reason snow is still falling. Marisa's Detective Agency (Marisa, Alice, Patchouli, Flandre) has been holed up in the Hakurei Shrine, and Reimu wants them to stop freeloading, so she kicks them out and tells them to go find out why it's still snowing. However, before they leave Mokou & Keine show up and request that the Detectives go find Mokou's "fiery core" that has gone missing. Mokou, who can't fight without her core, decides to go with the girls, and leaves Keine at the shrine with Reimu as a "registration fee".