Touma Kamiyama (Kamen Rider Saber) & Mei Sudo meets a girl (Kanon Fukami) who is searching for her brother (Makoto Fukami / Kamen Rider Specter). While they were talking with her, she gets kidnapped by Desast. Why was she kidnapped by Desast? Touma & others search for Desast & the girl.
On the other hand, Takeru Tenkuji (Kamen Rider Ghost) who was searching for Kanon comes to know about remnants of Danton. He doesn't anyone to get sad just like Chloe. He runs after the remnants of Danton & crashes with Touma, Mei.
Danton (Evolude) & Chloe are two characters that were introduced in Ghost RE:BIRTH: Kamen Rider Specter who are the father & sister to both Makoto & Kanon respectively.
Since Makoto gets referred in the Spin-Off, there could be chance that Makoto Fukami might be the unannounced surprise character of the spin-off.
Mio Kudo version Special "Kiramager Vs Ryusoulger" & "Kamen Rider Saber × Ghost" Trailer