Manny and Sid must risk life and limb to rescue their buddy Sid from his new hostile environment after he accidentally adopts a nest of dinosaur eggs. The gang soon realises that Sid has gone AWOL and they are going to have to journey into the perilous lost world populated by dinosaurs beneath the ice. Along the way, Manny and Diego contend with Manny's desperation to make life safe for their unborn child, while also dealing with Sid's anxiety and a mid-life crisis faced by Diego, all set against the backdrop of a vast subterranean jungle inhabited by dinosaurs.