Arriving in the World of Kiva, Tsukasa and company discover that the Fangires of this world have made a truce with the humans. However, when Tsukasa finds the Lion Fangire attacking a young woman, he transforms into Decade to fight him. Wataru, the King of the Fangire Race, witnesses this and becomes Kamen Rider Kiva to fight him with the aid of his Arms Monsters, forcing Decade to assume Kamen Rider Kuuga's Mighty, Titan, and Dragon Forms to counter. Yusuke manages to stop the fight, and the Swallowtail Fangire reveals that the young woman was actually a renegade Fangire. Yusuke reveals he has promised to help Wataru because he is uncomfortable with being the King. After meeting a mysterious man who is disgusted with the human/Fangire treaty, Tsukasa later encounters Kiva-la, who uses her power to send him into another world where he is attacked by Kamen Rider Kaixa. Back in the World of Kiva, after having fed on many humans, the Beetle Fangire arrives at Castle Doran with the intent to kill Wataru and take his place as the King.
Recap / Kamen Rider Decade Ep 4 Second Movement Prince Kiva