Scrat is once again trying to bury his acorn but accidentally activates an abandoned alien ship that takes him into deep space, where he unwittingly sends several asteroids en route to a collision with Earth. Meanwhile, Manny and Ellie are preparing upcoming marriage between Peaches and her fiancé, Julian. Diego and his wife Shira want to start a family, but their fierce appearance tends to scare kids. Sid is dumped by his girlfriend, Francine, just as he is about to propose to her, and he laments his solitude. During Manny and Ellie’s wedding anniversary party, some of the asteroids strike the place and The Herd barely escape. Meanwhile, at the underground lost world, Buck returns a dinosaur egg back to its rightful owner after it was stolen by a trio of flying dromaeosaurs named Gavin, Gertie, and Roger. Buck discovers an ancient stone pillar and takes it to the surface, where he meets Manny and the others.
Buck explains to The Herd that according to the pillar, the asteroids had caused several extinctions in the past and with a massive one still incoming, he believes that the only place they could find a clue to stop it is on the site of the impact of the previous ones, as according to its engravings, they always fall at the same place. However, the three dromaeosaurs overhear their conversation, and Gavin and Gertie decide to stop them, believing that they could easily avoid the impact, thus not only getting their revenge on Buck, but also killing all mammals and securing domination over Earth for their species. Roger is reluctant, but Gavin and Gertie strong-arm him into cooperating.