It is an original story taking place during the Persona 4 storyline shortly after Yukiko Amagi's rescue. The story is primarily focused on Yukiko, Yosuke Hanamura, and Chie Satonaka as they enter the television world without the protagonist, who is written out of the scenario by texting the others he won't be able to make it that afternoon.
Yosuke, instead, takes the lead role in this story. The premise is largely about him entering a different television than usual, drawn by a mysterious voice and the familiar figure of Saki Konishi.
(Source: Megami Tensei Wikia)