It was a pre-destined love and marriage for Sonoko and Tetsuo. They tied the knot and became husband and wife no questions asked. All is well then. Well, perhaps not. Each holds a secret that even the binds of matrimony cannot untie.
Shiino one day finds out that her best friend Mariko has committed suicide. After snatching Mariko's urn from her abusive father, Shiino heads to Marigaoka Cape, a place Mariko always wanted to go.
The gears of fate set in motion by the death of a young friend in their youth. A composer, tormented by the intertwining of love and friendship, struggles to find the 'Promise Sonata.'
