Not long after sending out a radio signal, the Robinsons' radar shows a spacecraft heading in on them. They find the vessel is originally from Earth, piloted by the renowned astronaut Jimmy Hapgood, who disappeared on a mission to Saturn many years before. Hapgood, attracted by the prospect of interstellar space, abandoned his mission and has been rambling among the stars ever since.
Will is very impressed meeting a real hero and quickly offers to help Hapgood decontaminate his ship. Over the next few days he spends a lot of time with the astronaut and Hapgood is pleased to regale the boy with tales of the many amazing things he has seen in his travels.
After John and Don help Hapgood make repairs to his spaceship, John and Maureen consider asking Hapgood to take Penny and Will with him to Earth. The children are distraught at the idea, but agree. When the Robinsons broach the idea, Hapgood asks for time to think about it, and when he declines, he gets into a fistfight with Don. Even Dr. Smith attempts to talk Hapgood into taking him to Earth, but he too is refused. When Hapgood returns to his ship, he finds it overgrown with spores, it not having been properly decontaminated at his arrival. When Penny is almost killed by the spores, Hapgood realizes the dangers the Robinsons face and agrees to return the children to Earth for their own safety.
Dr. Smith is furious when he learns this. Out of spite (and to have the chance to replace the children on board Hapgood's ship) he begins to play on their renewed sadness at the prospect of leaving their family. He is quickly able to trick them into running away. Since Hapgood must launch at a specific time or risk being marooned with the Robinsons, all the children have to do is remain away long enough. The family searches for them, but despite their best efforts, Hapgood must take off without the children.
- A "landing on Saturn" is not a mistake in the context of 1965. At the time, scientists believed that even Jupiter had a discrete surface. Since the Pioneer probes passed the planet in 1973 and 1974, it has been understood that the four gas giants do not have a solid surface, although they have a rocky core surrounded by gases under such immense pressure that they form a slush. If this had been understood in the early 1960s, Hapgood's course might more likely have been a landing on Saturn's moon Titan. However, one must surely wonder what Alpha Control was thinking when it sent out a small, one-man spaceship on such a mission. It is possible that a misfire or something similar to the Jupiter 2's experience blew it off course, but Hapgood's explanation - that he just felt like taking off someplace - is strange. One wonders how he managed to even be picked for the mission.
- This is the second episode in a row where the Robot is disassembled.
- This is one of the last episodes in which John and Maureen kiss.
- Hapgood left Earth at 6 AM on June 18, 1982. Will says this is "more than 15 years ago". The Robinsons launched in 1997.
- Hapgood's spaceship is hardly larger than a Gemini capsule. One wonders if it docks with a larger craft left in orbit, as well as where it stores fuel, propellant, air, or food.
- Will says that Earth has a polluted atmosphere. This is hardly surprising given the population explosion of 1997, however, in the later episode "Return From Outer Space" little seems to have changed!
- Hapgood claims he was "coming in on the Epsilon orbit" when Will interrupts "Epsilon Indi?" Hapgood replies, "No, Eridani." Both are type K main sequence stars, as is Alpha Centauri B. A 1964 report by the RAND Corporation entitled Habitable Planets for Man listed Epsilon Eridani as a star with a relatively high 3.3% chance of having a planet worthy of colonization, so this exchange was in line with the best knowledge of the time. Extrasolar planets are claimed to be present around the star, and it has even been the focus of a SETI project.
- Smith claims to have doctorates in medicine and science. It is known he is a Space Psychologist and presumably he has enough medical knowledge to be employed by Alpha Control as a physician, although the Alpha Control Reference Manual mentions many of his credentials are forged.
- Hapgood takes care to wear gloves while decontaminating the exterior of his ship of alien microbes but lets Will climb on it with his bare hands.
- Hapgood sees a meteor upon his return to space, but he calls it a nova.
- The footage of Hapgood's capsule leaving the planet would be used for close to every alien spaceship's departure throughout the rest of the show, whether or not the ship bore any resemblance!
- The rockets on the bottom of Hapgood's ship show shooting flames as it descends towards Priplanus. Assuming the ship works as conventional rocket technology does, there should be something more akin to a standard jet exhaust as the pressurized heated gas in the reaction engine escapes into the lower pressure environment of space or Priplanus' atmosphere. Flames could occur if the ship was in an oxygen-containing environment, the exhaust tubes were hot, and a combustible material leaked onto them and ignited. Oddly the way the thrusters on the Jupiter 2 are shown to work is fairly close to reality.
- Hapgood was an American Astronaut who became "Lost in Space" in 1982, but Professor Robinson, Major West, and Doctor Smith don't recognize him!