Die ISDA sucht nach dem Originaldrachen Ostrum, der untergetaucht ist. Währenddessen gibt Professor Kitajima sich nicht damit zufrieden, auf Widow und Gio aufzupassen.
The original fourth dragon, which the ISDA name "Ostrum", wreaks havoc on Earth and then suddenly disappears before the ISDA can take action. Kasuga informs the ISDA that Ostrum has resonated with someone, but he does not know who. Ostrum later reappears, and enters into battle with the Lindworm Unit, but Ostrum overpowers them. Akira, who's watching from afar, decides to help her comrades, and joins in the battle with Machina. Meanwhile, Jin is helped by Kō, who takes him to Tartarus. Upon their arrival, Jin is obstructed by Kazuki, who still claims that Gio belongs to him. Back at the ISDA base, Kasuga is confronted by Yuuri. After refusing to answer her questions, he proceeds to have the computer drop the container holding the ISDA's original dragon sample. Ostrum, who had succeeded in losing the Lindworm Unit, appears in his Communicator form, and asks Kasuga why he decided to side with humans.
タナトスからの新たな使者の到来に出動するドラゴノーツ達。だがそれは、ドラゴノーツ達の目の前で、忽然と姿を消してしまう。不安とパニックに見舞われる新小笠原全土。
全島を封鎖し、ドラゴノーツによる必死の捜索活動が開始されるが、手掛かりも無く、発見できないまま時は過ぎる。
そして、タルタロス島で拘束されているギオの元を訪れる人影。絶体絶命の危機に晒されるドラゴノーツ基地。ドラゴノーツは果たしてこの危機を退ける事が出来るのか？
مع الأصل الرابع طليقًا، تقوم (و.د.ت.ف) بإجلاء المدينة وإرسال وحدة ليندوورم
