John and Don are capping a hole they were drilling, they take cover behind a large rock. Just as Don hits the timed detonator, Penny walks by calling for John, he leaps out from behind the rock and tackles Penny just as the explosion goes off. They both seem to be okay. Penny wants to stay with them to help, but John sends her to find something else to do. Penny goes off exploring the area by herself, She stops by a lake and looks at her reflection in the water, a fountain of water spring up than dies away. She jumps back yelling "Hey", she hears an echo and follows the sound to a cave. She talks again, the echo sound like a male voice.
Back at the ship Judy picks a new hairstyle as Maureen puts the hairstyling machine on Judy's head. Penny comes back and tells will she has a new friend, but he is only an echo. Will teases Penny about her imagination. Penny tries to explain to Maureen and Judy about the place she has found and her friend, but they don't pay her much attention. Judy's hair style is changed within one minute of using the hairstyle machine. Penny walks off unnoticed and back to the cave.
Penny figures out that the voice is not just an echo. She begins asking her friend questions. Maureen and Judy bring lunch to John and Don at the drill site. Meanwhile, Smith plays a game of chess with the Robot as Will observes. Will sees penny in the distance and runs up to talk to her. Penny is playing a game of Jacks, Will asks where she got them from, she says her friend gave it to her. Will picks up the jacks and says they are made of crystal. He tosses them and they land on the chess board. Penny says her friends walls are fills with them. Smith notices that the crystal are actually diamonds, he runs off after Penny. He tells her he would like to meet her friend. She refuses saying he is shy and only wants to talk to her. She walks off. The Robot confirms Smith's observations, the crystals are diamonds.
Penny goes to her friend's cave, she is followed by Smith and the Robot. A heavy rock blocks the cave entrance stopping Smith from entering. Penny has a book to read to her friend. She tells him that will says he is nobody.
Penny tries to explain the meaning of death. She begins calling him Mr. Nobody. Smith finds a hole in the cave ceiling and talks to Penny pretending to be Mr. Nobody. He tries to get her to collect a bunch of large diamonds. The hole opens a bit and Smith gets wedged in the hole. Penny helps Smith out of the hole. Smith decides to blast a hole in the cave.
Smith gets Don to help he with the explosives. Penny wants to go back to the cave to talk with Mr. Nobody. Maureen says it is too late, that Penny can talk to him right for the Jupiter 2. Penny is upset that Maureen still thinks Mr Nobody is imaginary. Don and Smith set up explosives outside the cave. Will tells Penny he saw Smith and Don heading out with the Drilling equipment. She figures out that Smith is after the diamonds. Penny sneaks out and head to the cave to warn Mr. Nobody. Don sets a plastic explosive pellet into the drilled hole. As Dons sets up the detonator, Smith drops a handful of explosive pellet into the hole. Penny enters the cave. Don sets off the explosives, the cave ceiling collapses, Penny is knocked out.
Mr. Nobody gets very angry thinking that Penny is dead. Mr. Nobody decides to go outside the cave to find whoever is responsible for the cave-in. Mr. Nobody causes a huge storm to kick up. The ground violently quakes. Maureen finds that Penny is not onboard the ship. Judy and John run to help Don who fell while heading towards the ship. Everyone gets into the ship, they send the Robot to discharge the force using an electrical field. Penny wakes up and heads back to the ship. Penny tells Mr. Nobody not to hurt anyone. The storm breaks. After feeling Penny's love Mr. Nobody dissipates. Will finds the Robot in many pieces.
The next day Will finds the Smith has repaired the Robot. Unknown to them an alien is viewing the Jupiter 2 camp site.