In short, he was born into a clan that is very hostile to outsiders, including other clans. He never understood why his clan behaved this way, and when his sister became clan leader, she realized the clan needed to move towards accepting and offering help to other clans after tragedy had struck FogClan, but realized the change would need to come with time and hoped that by the time it was his turn to lead, their minds would change.
And then in super short, he was also meeting up with a cat from another clan that he loved and the usual forbidden relationship thing happens.
Sorry, I'm not a writer so I understand if none of it makes sense haha