Cardfight!! Vanguard Gaiden: If

Upon the conclusion of the Vanguard High School Koshien, Kouji Ibuki begins to have new nightmares.

Nome Tatsunagi has Suiko summon him to the mansion where they learn he can see a Dark Tower called Sanctuary.

Sanctuary is causing the timeline to be altered throughout history, and the only way to correct it is for Kouji to first correct his future mistake.

Now Kouji and Suiko must travel the parallel world -if- and watch as Magical Girls Emi Sendou and Shuka summon Vanguard units to help correct history.

Sadly, the two can't help much, as Cardfighting against real monsters doesn't do much good.