Cardfight!! Vanguard Gaiden: If

After the intense events of the Vanguard High School Koshien, Kouji Ibuki is haunted by unsettling dreams. He is summoned to a mansion by Nome Tatsunagi, where they discover the existence of a mysterious Dark Tower known as Sanctuary. This tower is responsible for disrupting the timeline, and to restore balance, Kouji must first confront a significant error from his own past.

Joined by Suiko, Kouji embarks on a journey through the parallel world of -if-, where they encounter Magical Girls Emi Sendou and Shuka. Together, they engage in Cardfights with Vanguard units in a bid to mend the fabric of history. However, the challenges they face are daunting, as battling against actual monsters proves to be more complicated than anticipated.