Zardoc's goal is to get humans accepted into the greater multiverse, which will require a major adjustment of the human psyche. With Jayna and Makenna's help, the goal has been in the distance, but in sight and moving closer all the time.
With an impending environmental disaster brought on by massive denial and big money interests, the original plan needs serious adjustment. Instead of helping humanity to be accepted, now they have to save it from itself.
Follow the Castaway Island group as they come back to the Earth once again, to cruise around for a short vacation before Zardoc and Jayna have to work on their plan, which will allow the Earth to recover from the abuse of human occupation.
They find new friends, have new interesting experiences, and go an all-new direction.
Will Zardoc and Jayna come up with a workable plan that covers all the bases? What is it like being considered ghosts? How are all of the necessary changes going to be made? Will the Earth survive? Will the human species survive?
Interesting questions, all dealt with in the Castaway Island series.
Sure, it's Science Fiction, but nothing points to future Science Fact like good Science Fiction.
It's just the way it is.