Well I would give a personal opinion on that.
Step-by-step explanation:
Sir John Franklin meets a terrible death and this leads Captain Francis Crozier to take the lead of the program. They are desperate for their survival and they know that that is what is remained for them. A young lady is there with Captain Franklin, she might help them all to fight this hardships and find a way out for survival. They have a very mysterious enemy that is constantly behind them, this is a part of the suspension the author have kept.
Now coming to the Central Idea part, which is that whatever happens whatever comes your goal should be your survival, the world is a adventure, our lives are an adventure and we are given to survive here, the one who can survive is the winner.
(You can use the conversations between the cree and the Captain's inspiring speeches to support the answer.)
Hope this helps....