Saa, Docchi is a halloween song composed by Hinata Haruhana. It was made to celebrate Halloween and also to enter the -Project DIVA- Arcade submission. Later Saa Docchi was chosen to be featured in the game -Project DIVA- Arcade and is also featured in PDA's OST album.
On late 2011, the append version of the song (Len Append cold and Rin Append warm) was uploaded to Hinata Haruhana's Piapro account. Earlier, it was only featured in Hinata Haruhana's album Mechanical Mystery.
This song has no definite plot, it tells of two people who have different point of views. The main question was "Trick or treat" and Rin and Len replied differently. Len answered "trick" and expects you to give him an impossible riddle. Rin answered "treat" and expects you to give her a treat, or else you'll be eaten.