The Pevensies left Narnia, leaving the king who had just ascended the throne alone. Caspian seemed to have finally found the family that he had been missing for so long, but these people left him as quickly as they appeared in his life.

It's been a little over a year since his friends left. Caspian was sad, tried to forget himself, but never tried to find company equal to his young friends. he knew that there was no equal to them and there would never be one in this world. the king had to accept that he would never see Pevensie again.

he needed somebody because he had lost somebody.

(then you can come up with a plot yourself)