It takes place 20 years in the future. Magic seems to be coming back unpredictably. Starfan (now in her late 30's) is now a Nefcy insert and the season is about her mentoring Star and Marco's 14 year old son/daughter after his/her parents mysteriously disappeared. Starfan is agreed to be the best one to do this because Starfan chronicled all of Star's adventures.
It delves into /ss/ or /ll/ pretty quickly, explicitly expressing how wonderful it can be to have an older, more experienced adult train you in the ways of love. >pic unrelated