It's the beginning of the Ice Age in prehistoric times and herds of animals are migrating to avoid the oncoming and permanent wintry weather. That is, except for daffy Sid the Sloth (voice of JOHN LEGUIZAMO) who's overslept as usual and has been abandoned by his family.
After a bad run-in with two ill-tempered rhinos (voices of CEDRIC THE ENTERTAINER & STEVEN ROOT), Sid flees for his life and then runs into Manfred the Mammoth (voice of RAY ROMANO) who's heading against the flow of traffic. Manfred's sheer size allows him to defeat the rhinos' attack and that makes Sid think he's found a new friend. Manny wants nothing to do with the little fellow, though, and makes no bones about that, but Sid won't let him be.
Meanwhile, a group of saber-toothed tigers -- Soto (voice of GORAN VISNJIC), the leader, and his followers, Diego (voice of DENIS LEARY) and two others (voices of DEIDRICH BADER & ALAN TUDYK) - are planning an attack on a tribe of humans they claim has killed half of their pack. Hungry for revenge and dinner, Soto orders Diego to seize a toddler and return it alive to him. The tiger attempts to do that, but the child's mother manages to escape, but without ultimately sacrificing her life.
The human child eventually lands in the possession of Manny and Sid, with the former wanting nothing to do with it until Diego's arrival and statement that he simply wants to return the baby to its tribe. Not trusting the tiger, Manny decides to return the toddler himself, but Diego warns that without his tracking abilities, the mammoth, sloth and the toddler might perish in the harsh environs.
From that point on, and as a saber-toothed squirrel, Scrat (sounds of CHRIS WEDGE), repeatedly comes across their path all while trying to protect and bury his precious acorn, the unlikely trio sets out to return the baby, with only Diego aware of what lies waiting for them at the end of their journey.