The majority of the movie, oddly, takes place in Fuuto, but Philip is nowhere to be seen. This point is mostly ignored. Shotaro’s introduction is said by DizzyDennis to be “hilarious and extremely fitting to W.” Even Fuuto-Kun apppears!
The plot involves 2 stories. The main story involves the clash between the Heisei and Showa era Kamen Riders. While DizzyDennis doesn’t give specific details, he mentions it isn’t thoroughly explained, but basically boils down to a “Clash of Pride.”