Sue convinces her family to take the day off. However, as they walk through the streets, Reed dedicates much of his attention to his work and Johnny obsesses about the arrival of his new car. When Ben picks a fight with Johnny, Susan is forced to break them up. They just start to agree to enjoy themselves when Giganto, an enormous subterranean monster, breaks through the street and goes on a rampage.
The monster grabs Susan and the team moves to help her. She manages to pry herself free while Ben ends up ground into the pavement when Giganto steps on him. The creature prepares to attack again, and then stops, sniffs the air, and strides off toward the Baxter Building. Realizing his new car is in danger, Johnny flies on ahead and tries to stop the monster with the aid of Ben and Susan. Reed goes into his lab and gets a stasis gun to paralyze Giganto. As the crowd cheers them, Reed suggests that they investigate the tunnel that Giganto came through and find out what's going on.
The team descends into the depths and finds a cave filled with diamond stalagmites and waterfalls of liquid helium. Before they can proceed further, hundreds of Moloids attack the team. Ben is caught in a puddle of the creatures, Reed is surrounded and locked into place, and Johnny is frozen with liquid helium. Susan tries to use a force field to protect herself, but a Moloid emits sonic waves to penetrate her shield and then gasses her unconscious. When she wakes up, she discovers that the Mole Man has arrived to boast of his triumph. He shows them a cave filled with Giganto's eggs and explains that when they hatch, they will imprint on the first thing they see. Since he has sent Giganto away, they will imprint on him, obey his commands, and attack the surface world. Reed insists that they'll stop him, but the Mole Man notices that they are helpless against his forces and the creatures can hold Susan since they don't rely on sight.
After the Mole Man leaves, Sue slips free and leaves a force field in her shape to replace her: the blind Moloids don't realize the switch. She cracks open Reed's mud shell and they realize that one of the eggs is missing. Reed realizes that the Mole Man hid it in the Baxter Building to lure Giganto there. He tells Susan to get the egg and bring it back to the cavern so Giganto will follow it there and recover its eggs. She slips away and the others distract the Moloids after the creature realize Susan is gone.
Susan returns to the Baxter Building as Giganto starts to free herself. She finds the egg in the garage just as it hatches, and a baby monster emerges. Susan renders herself invisible so it won't imprint on her, and it runs outside to find its mother. The two are reunited, and Susan decides to take Johnny's new car and get Giganto back to the tunnel. She puts the baby in the car with her and drives for the tunnel.
In the caverns below, the Mole Man returns and realizes that Susan has escaped. Reed finishes freeing himself and then frees the others, and they battle the Moloids. Johnny takes time out to call and see how his car is doing. Skidding through the streets, Susan reassures him that everything is fine. She drives down the tunnel toward the cavern, while the Mole Man prepares to let the emerging hatchlings see him. Reed and Ben come up with a plan for the hatchlings to see Ben first, but Susan arrives with Giganto in pursuit. It steps on Johnny's car and recovers its child. Thinking quickly, Susan renders the Mole Man invisible so that the hatchlings can't see him. They find their mother and imprint on her, and Susan makes the Mole Man visible again. He quickly begs her to make him disappear when Giganto starts attacking him. The team finally agrees to help and Johnny distracts the monster while Reed captures the Mole Man. Giganto seals off the tunnel, protecting itself and its children from any further outside interference. As the Fantastic Four take the Mole Man way to the Vault for safekeeping, Johnny complains that he wanted to wreck his car. Susan promises to let him trash the next car he buys.new