20,000 years in the past, we encounter a rather grumpy mammoth named Manfred alongside an overly enthusiastic ground sloth named Sid, both left behind during their migration amidst the harsh ice age. After being abandoned by his own family, Sid attempts to venture out on his own but quickly finds himself in a precarious situation with his two rhino friends, Carl and Frank, who initially try to chase him off. Just as it seems Sid might be in serious trouble, Manfred steps in to protect him, leaving Sid feeling grateful but also determined to stick close to the mammoth, much to Manfred's annoyance.
Nearby, a group of humans camps by a waterfall. Among them are their leader, Runar, his wife Nadia, and their baby son Roshan. Meanwhile, Soto, the malevolent leader of a pack of saber-tooth tigers, observes the humans closely, plotting his revenge for their role in decimating his pack. Soto's dark plan takes shape as he aims to capture Roshan and use him to pay back the humans. The following morning, a chaotic attack unfolds, during which Nadia manages to escape with Roshan by leaping over the waterfall, leaving Soto's lieutenant, the cunning Diego, empty-handed.
As fate would have it, Manny and Sid soon encounter Nadia on a riverbank, weak and desperate. In a fleeting moment, she manages to pass her child to Manny before slipping away from exhaustion. Despite Sid's insistence on returning the baby to his family, Manny is reluctant but eventually agrees to help, especially after realizing Diego's true intention of returning Roshan to Mankind. Together, the unlikely trio sets forth on a journey north toward Glacier Pass, where Runar and the other humans wait.
On their way, they meet an eccentric group of dodos, which leads to a comical chase for food. However, Diego has ulterior motives, as he is secretly in touch with his saber pack. Despite the challenges they face—including intense weather conditions and encounters with the notorious saber-tooth squirrel Scrat—the bond between Manny, Sid, and Diego begins to strengthen.
The journey takes them through an ice cavern adorned with ancient paintings, revealing Manny's traumatic past and the loss of his family at the hands of humans. In a poignant moment, Roshan reaches out to a painting depicting a mammoth family, providing Manny with the comfort he desperately needs. As they get closer to their destination, Diego's moment of betrayal looms, but he grapples with his choice and ultimately reveals his plan to ambush the others.
Complications arise when a confrontation occurs with Soto's pack, leading to a fierce battle. As Manny saves Diego from impending doom, a transformation occurs within Diego, who chooses to stand by Manny instead. In a climactic showdown, Soto falls to his own trap, leaving Manny, Sid, and Roshan safe once again.
The trio finally reaches the humans, where Manny bravely presents Roshan, diffusing the tension and gaining the group's gratitude. In a heartwarming conclusion, Runar expresses his appreciation with a token of honor before the humans depart. With Diego surviving the ordeal and reuniting with them, the three friends—Manny, Sid, and Diego—decide to forge ahead as a new herd, ready for whatever challenges lie ahead.
