Broadly speaking, this film tells the lives of animals that are disrupted by climate disasters. A climate disaster can change everything. Including the comfortable life of Sid the sloth, Diego the ancient tiger, and Manny the mammoth who had just been happy after finding a mammoth female partner. Great earthquake makes the ice sheets they live in crack.
What happens next? The three of them separated from the main ice plain and floated on an ice sheet until it was carried by the open sea.