Tsukasa finds himself wearing strange clothes with another strange card as he, Natsumi, and Yusuke find themselves attacked by the Strange Demon Robot Schwarian and his Chaps as Kamen Rider Black RX battles them, with both sides believing Tsukasa to be Joe the Haze. When the Seamoon Fangire and Scorpion Imagin suddenly appear, Tsukasa becomes Decade to help RX out against the monsters with Agito's power as Schwarian runs off. After Natsumi stops RX from attacking Decade, RX takes her words to heart and sees Tsukasa as an ally.

Following Apollo Geist into another A.R. World, Decade is attacked by Kamen Rider Black until Diend arrives and summons Femme to hold him off. The next day, after learning that he is in the World of Black and seeing that he is on his own in saving Natsumi, Decade ambushes Guy as Ryuki and fights him until he uses a little girl as his hostage. When Decade takes a hit from Diend to save the child, Black watches and has second thoughts about him as Apollo Geist escapes.