Tyrone's lucky penny falls way down to the center of the Earth, and Professors Uniqua and Pablo use their inventions to try and help him retrieve it.
Tyrone is in the sandbox digging. He tells the viewer he is looking for his lucky penny, which must have fallen into the sandbox. Tyrone's search is interrupted by Uniqua pulling Pablo in a wagon. The wagon glides in the air after hitting the sandbox. Tyrone tells the two to stop because he needs to concentrate when looking for his lucky penny. Uniqua tells him that she and Pablo will help. They sing Amazing Inventors to explain how they are inventors who invent contraptions to make life easier.
The backyard transforms into a dark building where Uniqua and Pablo's inventions are on display. Uniqua points to a paper showing the Earth. Uniqua shows Tyrone an image of Tyrone's penny in the middle of the Earth image as Pablo explains how his penny probably fell to the center of the Earth. Uniqua and Pablo sing a reprise of Amazing Inventors as they show Tyrone their many inventions. They bring him to a room where a large metal drill is set up. Pablo tells Tyrone that it is their rocket-drill (which is part rocket, part drill), an invention they will use to get to the center of the Earth.
The three hop into the drill and sing To The Centre Of The Earth as they drill deeper into the Earth. They stop at a spongy layer of the Earth. The drill bounces on the material. On a screen, they spot Tyrone's penny. Pablo is disappointed because the penny didn't fall all the way to the center of the Earth after all. They jump out. Tyrone carries a box of inventions out and leaves it on the ground as he rushes to the penny. He is stopped by Uniqua, who takes an invention out of the box. She tells Tyrone that they will use an invention to pick the penny up. Pablo tells Tyrone that the invention Uniqua is controlling is called the "grab-o-copter", a flying invention that will fly over to the penny and grab it.
Uniqua uses the machine. Its robotic claws grab the penny, but the penny falls and begins bouncing. Tyrone runs after it, but it falls into a chasm leading deeper into the Earth. The characters hop back into the rocket-drill and sing To The Centre Of The Earth again. They stop when they spot Tyrone's penny on top of a large, pole-shaped diamond in a diamond layer of the Earth. They all get out. Tyrone puts the box of inventions on the ground and runs to the tall pole. Uniqua stops him and tells him again that they will use inventions to retrieve the penny.
Uniqua gives Pablo shoes with toilet plungers on the bottoms of them. She calls the inventions "Vacuu-Shoes". Pablo uses the shoes to climb the pole and reach the penny. Tyrone asks Uniqua why he couldn't of just shaken the pole so the penny would fall into his hand. Uniqua ignores the question. Pablo accidentally causes the penny to fall and slide on the diamond-covered ground. Uniqua, Pablo, and Tyrone sing The Diamond Slide as they try to catch the sliding penny.
The penny falls into a small hole. The inventors and Tyrone walk back into the rocket-drill and sing To The Centre Of The Earth as they drill deeper. They reach a small island-like formation in the middle of a large moat. Pablo exclaims "We've reached the center of the Earth!" as the three walk out of the rocket-drill. Uniqua spots the lucky penny in front of a large green dinosaur. Because all of the characters are afraid to walk over to the dinosaur and grab the penny, they use the inventions in the box to make an invention called the "extendo-arm", which will be able to snatch the penny from under the dinosaur.
They sing The Extendo-Arm. They try to use the arm to retrieve the penny, but the dinosaur notices and runs after them. They run for their lives to the rocket-drill, but the drill sinks into the water. Tyrone saves himself and his friends by playing "fetch": he throws the penny and the dinosaur runs after it. The dinosaur brings the penny back in his mouth and gives it back to Tyrone. The three notice that the dinosaur is friendly. Tyrone invites the others to his house for sweet potato fries. The center of the Earth transforms back into the backyard as Uniqua, Pablo, and Tyrone sing the end song. They run into Tyrone's house. Tyrone opens the previously closed door and exclaims "My penny!".