Peaches is set to marry a kind-hearted mammoth named Julian, a match that doesn't sit well with Manny, who finds Julian both annoying and unfit to look after Peaches. Tensions rise when Peaches expresses her desire to travel the world for their honeymoon, a notion met with disapproval from both Manny and Ellie. Meanwhile, Diego and Shira are contemplating starting a family but worry that their offspring might struggle to make friends due to the fears surrounding saber-tooth tigers. Sid, on the verge of proposing to his girlfriend Francine, finds himself heartbroken after being dumped and reflects on his loneliness.
During the celebration of Manny and Ellie’s wedding anniversary, the festivities take a dramatic turn when asteroids, sent hurtling towards Earth by Scrat's galactic misadventures, rain down on the herd. They narrowly escape the chaos. In a parallel storyline, Buck bravely returns a lost Chasmosaurus egg to its mother after it is taken by a trio of flying dromaeosaurs—Gavin, Gertie, and Roger. Upon discovering a mysterious ancient stone pillar, Buck brings it back to the surface, reuniting with the herd.
According to Buck, the inscriptions on the pillar suggest that past asteroid strikes led to two previous extinctions, and he believes that the impending asteroid presents an opportunity to find clues at the site of these ancient impacts—specifically, a nearby volcano. However, the cunning dromaeosaurs overhear their discussion and, eager to avenge themselves on Buck, plot to thwart the herd’s mission while aiming to eliminate Earth's inhabitants to ensure their dominance.
As they journey towards the crash site, Buck devises a plan: if they can gather enough small asteroids and launch them into orbit, they might be able to divert the hazardous asteroid from its collision course with Earth. Faced with numerous challenges and the constant threat of the dromaeosaurs, they eventually stumble upon "Geotopia," a mythical community of ageless animals that reside within one of the ancient fallen asteroids. Here, Sid encounters Brooke, a charming ground sloth, who is smitten with him.
Shangri Llama, the enigmatic leader of Geotopia, initially denies Buck’s request to utilize the community’s crystals to avert the asteroid's path, as these crystals are integral to their immortality. However, in a twist of fate, Sid accidentally wreaks havoc by trying to present one crystal to Brooke, which leads to the unwanted aging of the Geotopians, revealing their frail appearances and inciting the ire of Shangri Llama.
After Sid's realization, he persuades the residents that averting the asteroid takes precedence over their fear of aging. This leads the Geotopians and the herd to fill a volcano with a massive orb composed of the now-elderly crystal shards, creating enough pressure to launch it skyward, redirecting the oncoming asteroid. The dromaeosaurs attempt to intervene but, facing their obliteration from the impact, they reconsider. Buck manages to convince them that harmonious coexistence is possible if they work together.
In a nail-biting moment, Gavin drops the most substantial crystal just as it is about to be launched. Quick-thinking Manny and Julian partner up to recover the crystal, leading Manny to finally approve of Julian. Ultimately, the volcano erupts, propelling the crystals into space and safely diverting the asteroid.
As the herd heads home, Sid must bid farewell to Brooke, but unbeknownst to them, a magic crystal shard lands in a hot spring, rejuvenating the Geotopians and restoring their youth. Upon returning home, Manny and Ellie reconcile with Peaches, empowering her to pursue her dreams. In a joyous conclusion, Peaches and Julian wed, while Diego and Shira earn the admiration of the younger animals. Just then, a revitalized Brooke appears at the wedding, reuniting with a delighted Sid. Meanwhile, Scrat's antics land him on Mars, where his chaotic clumsiness results in the extinction of all Martian life before he awkwardly departs in a UFO.
