"trick and treat" is OSTER project's first Rin and Len duet song. It was uploaded for Halloween 2008. OSTER project's works are usually cheerful-sounding, but "trick and treat" was her first work to have a darker atmosphere. The song reached one million views on November 21, 2011, making it the second Kagamine duet song ("Gekokujou" being the first) to do so.
"trick and treat" basically tells a story of "you", who was led by Rin and Len into a deep, deep forest. Although they seemed friendly at first, "you" realized things were not as they seemed.
A fanmade PV that is a bit less dark and with a more cute approach by 7:24, and probably the only one with a "happy end". Miku get lost in the woods and runs into Rin and Len. They approach her with deceitful intentions that end being more childish that it seems in the beginning.
On October 1, 2012 it was announced that along with Ichika and Kurousa-P's Cantarella manga adaptation, trick and treat would also receive a manga adaptation on Nico Nico Age Premium Vol. 15; illustrated by NEGI. Chapter 1 was released on October 9, 2012 for 85 yen.
The story opens with Miku visiting Neru the shop keeper. Miku asked what the scroll above the shop means. The scroll reads "deep in the forest there lives a pair of devils". Neru explained how there was a rumor going on, and how someone who lived in the city went missing because of the "forest incident". They changed the conversation completely as Neru offered her a new pair of ribbons with a discount.
Miku went home late and had to go to the forest on her way there. The forest was dark and she admitted Neru's story scared her a little bit. It was then when she heard of children's voices which lead her to a vast mansion. Two children then surprised Miku by sneaking up behind her, and then asked her if she was lost. The pair of children apologized for startling her and welcomed to "a game of secrets (place)"; meaning their "mansion".
Once they were inside, they introduced themselves as Len and Rin. They appeared to be outgoing and innocent, much like any other children around their age. Miku gave them cookies, which made the pair like Miku even more. As a thank you gift for the cookies, Rin gave her a blindfold. She explained that the mansion there was a "secret place" that no ordinary person can access. Since Miku had somehow made it to the mansion to "play", they figured if Miku wants to go back to the mansion and play again she should just put on the blindfold and it will lead her back without going through the dark forest's "maze".
The second Miku put on the blindfold, she somehow arrived at Neru's place in a different place and time. The following day she decided to go back to Rin and Len's "secret place", and brought a cake along with her since they had loved the cookies before. As a thank you gift, Len and Rin introduced "cinnamon stick" (a reference to the song) with that they scooped a hand full of syrup and ate them with scones the two served. The scones somehow made them sleepy and tired; and so Miku stayed with them in the mansion for the night.
Miku enjoyed the stays and visits to Rin and Len's house, and brought a treat with each visit. She decided to learn more recipes, until Neru noticed how happy Miku had been the past few days. Neru brought up the topic of the rumors in the forest again, and for a second Miku was curious about how it was that the blindfold could lead her to that mansion.
Just for one night, Miku left bread crumbs in a trail behind her as she put her blindfold on and walked through the dark forest. Her blindfold was loose and there she saw two dark terrifying shadows which resembled Len and Rin, though it's nothing like their child-like figures. She let go of her blindfold to see the "normal" Rin and Len. They managed to change the subject by asking for some sweets. "Unfortunately" Miku had no sweets to give (because earlier she left bread crumbs all over to trace her tracks). Upon hearing this, Rin and Len became very upset and grabbed her.
The following morning, Miku woke up in her own room. She began to wonder just what Rin and Len were, and if they were really as innocent as they looked. She wondered if they were the twin devils she'd heard rumours about. The forest, the twins, and the devils (the shadows had seen saw not too long ago): they were all somehow connected with Rin and Len, though she refused to believe that.
Later on Miku found out their true nature, really being the devil twins to be rumoured around. The story ends the same way the song "trick and treat" ended, with "nee, choudai?".