Scrat the prehistoric squirrel/rat is back again chasing after an elusive acorn. He opens the film and is the last to settle in at the end. His sequence could have been a separate cartoon before or after the main film. He does fold in at the end but actions always fill us with laughter when he is on the screen.
The Meltdown is about ice turning to water around and below the many characters which at one point releases back to life giant creatures held in ice from way back. They are the villains who search for food in the pack of creatures who are fleeing the coming flood.
We get to see the paranoia of some of the characters who believe they are the only one remaining of their kind. It effects Ray Romano's Manny, the woolly mammoth, the most until he discovers Queen Latifah's Ellie who is obviously a woolly mammoth but thinks she is a possum because her life has been shared with Crash and Eddie (Scott and Peck), possums she calls her brothers. All three sleep hanging up-side-down by their tails in trees.
When Manny meets Ellie he is interested but bungles the relationship by making "save the species" as his reason for wanting to get together. It will take most of the film for the two to understand each other and to find hope for the future of woolly mammoths.
While Scrat steps into the picture from time to time and keeps up the humor and the laughs, Sid seeks respect and takes hits in the head on a regular basis, until he discovers a tribe of his own kind who see him as the fire king, which may be good or bad. The great cat Diego discloses his fear of water which is serious as the level of the liquid continues to rise throughout the story; in fact it may necessary for others to save the tiger.
The Meltdown, keeps us laughing at the slapstick comedy, even gives us a musical number which Busby Berkeley could have directed; it's not as solid as the original but it will keep all audiences laughing and an opportunity to laugh together with kids is rare and special.