Mermaid Man and Barnacle Boy decide to have lunch at the Krusty Krab. Learning this, Spongebob becomes overexcited and runs for an autograph, prompting the heroes to flee in fear and in the process, Mermaid Man drops his utility belt, which includes many features including a shrinking button, which Spongebob inadvertantly uses to shrink everyone in Bikini Bottom./Spongebob is in the process of failing his driving exam (yet again), and while they plunge off an unfinished bridge, Mrs. Puff imagines many outcomes of the event, one of which is going to jail.

With the impending ice age almost upon them, a mismatched trio of prehistoric critters – Manny the woolly mammoth, Diego the saber-toothed tiger and Sid the giant sloth – find an orphaned infant and decide to return it to its human parents. Along the way, the unlikely allies become friends but, when enemies attack, their quest takes on far nobler aims.