Shakugan no Shana S

Also known as:  Shakugan no Shana OVA

Shakugan no Shana S takes a step back from the ongoing series, focusing on the characters' back stories and "their deep relationships". The OVA has a total of four episodes releasing on Blu-ray and DVD, depicting scenes from Yashichiro Takahashi's original novel and related material from Shakugan no Shana. While searching through the hunters nest for a particular treasure, Shana and Yuji encounter some unforeseen difficulty when their minds end up being switched into each others' bodies. (based on ANN synopsis)

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Producers, official website and other details: AniDB, ANN, Wiki