This story is a sequel to The Last Vacation
Following the Battle of the Bands and their spectacular failure, Sonata and her sisters have isolated themselves in the mansion they 'acquired' long ago. Aria and Adagio, constantly fighting over who to blame for the failure, inevitably drag Sonata into their bickering. They ignore that which made her happiest: music.
Sonata, always the pawn in their power games, always the lesser sister, always too weak to assert her own will, leaves her sisters to their own devices for a day, trying to find some solace that might lessen the burden that's driving her slowly insane.
What she finds is one of the students they victimized: Flash Sentry, playing guitar in the park—alone.
This is a side-story to The Last Vacation, and reading it is not required to follow this story.