It is the year 2028. More than thirty years have passed since the historic launch of the Jupiter 2, and its brave crew has settled down into an isolated life on a planet many light years distant from Earth. Following their final crash landing some fourteen years earlier, the Robinson family, Don West, and Dr. Smith have lived a contented, if isolated, life on a planet with pleasant forests and blue lakes. Their vehicles and equipment are aging but still servicable, thanks to stored repair parts, specialized maintenance schedules, and Will's genius for improvisation. They have no idea where they are in space -- nor can they even begin to guess where Earth is.

Seven years past, John Robinson had died in an accident which also injured Smith. That day, three parties (consisting of Don and his son, Joshua; Will and the Robot, and John and Smith) had headed to distant, triangulated locations in order to set up an improvised radio telescope system in hopes of detecting some signal from space. The results of the terrible accident were not discovered until days afterward, when a worried Don and Will finally located the body of John and the near-death form of Smith.

Following his long recovery, Smith underwent a severe personality change. He quickly became like a grandfather to the Robinson children and grandchildren, and even Don finally succumbed and began to like the new, improved, reliable, honest Smith. His transformation was genuine -- the former saboteur was a new man.

One day, the Robinsons' lives change once more. A piercing signal is detected by the radio telescope system, a signal which repeats at regular intervals and is carried on a specific frequency used by Alpha Control back on Earth. It continues, weak but stable, for hours. Days. Weeks.

It is a navigation beacon, proudly singing out its location for all the universe to hear. Its location -- and that of Earth. Will remembers, as does Don, that there was to have been a 'Project Lighthouse' constructed near Earth as soon as interstellar traffic warranted. Don estimates that the beacon might have been built some ten years after their departure, based upon the plans of the time. If true, that means that, since the beacon's signal has been traveling at the speed of light, it has taken two decades to reach them. They are within twenty light years of home.

The Robinsons suddenly find a hope they had long since given up. Home is beckoning to them. Their energies renewed, they once more dedicate themselves to the repair of their only ride home, the Jupiter 2. For fourteen months, Don and Will create new ways of making the ship spaceworthy. Fuel is mined. Food is stored. Utilizing the Robot's electrical discharge as an arc welder, the hull is patched and reinforced. An abandoned, wrecked alien spaceship is salvaged for engine parts, and its components actually give them a propulsive system that allows them to travel faster than the Jupiter could when first launched. Everyone pitches in, especially Smith, and launch day slowly but steadily grows closer.

For the tenth time, the Jupiter 2 leaves a planet's surface. Its fusion drive roars with a determined power the Robinsons thought they would never hear again. As the ship's hull creaks with the strain of stressed metal, the ship reaches higher, higher, into the upper atmosphere and finally into orbit. After Will gets a solid fix on the beacon, the Jupiter heads out across the cold blackness of deep space, toward home.

The signal slowly grows stronger. For months the ship travels faster than lightspeed, driven as much by hope as by its deutronium fuel. Smith is the first to realize that the star patterns outside the viewports are beginning to coalesce into the constellations with which they are all familiar. Penny spots Orion's Belt. Judy recognizes Taurus. Their hope and excitement grows.

Finally, eighteen months after their trip began, a single yellow star grows larger and larger in the forward viewport. Soon, it dominates space ahead, and Will's spectral analysis shows it to be the Sun. The beacon's signal is loud and strong. As they happily watch, Saturn with her magnificent rings passes by, as, eventually, does Mars.

And then they see it -- a tiny blue speck, accompanied by a smaller white one. They grow closer. The planet's surface comes into view. It is Earth. They are home.

"If only dear Professor Robinson could have seen this moment," Smith quietly says, a tear welling up in his eye.

"He loved the Lord," Maureen states, comforting him. "I'm sure he's with us now. He knows his family's safe at last."

Don does the honors, picking up the microphone and calling Alpha Control. There is an amazed response from the other end, and Don is passed from Flight Control to the Space Director's office. They are understandably doubtful of Don's claim to be one of the Jupiter 2 crewmembers on orbital approach, but they finally believe him once he has given them a series of Jupiter 2 mission codes that only the Robinsons could know.

Will gets a fix on the Kennedy Space Center, home of Alpha Control, and they prepare for landing. As they break the clouds, they see that the space center is indeed there, plus the additional construction that one would expect to have occurred during the time they were away. They begin their final approach, accompanied by a squadron of welcoming chase planes.

After hovering over the launch complex for a moment, Don chooses an ironic landing spot -- the same launch cradle from which they had begun their adventure some thirty years earlier. It is empty, having been used a few days earlier by the most recent ship to leave the planet.

The engines shut down. The vessel goes quiet. A flurry of activity takes place outside as technicians swarm around the craft. Finally, Smith cracks the hatch to find the Flight Director just outside. He asks permission to come aboard.

Television cameras are trained upon the Jupiter's hatch, and an amazed world watches as the space pioneers emerge from the antiquated ship. There is applause from the ground crew as Don, Smith, and the Robinsons walk across the catwalk and into the ready room elevator.

The Robot is taken to the computer center so his taped information banks can be downloaded. The technicians are amazed that one of the old B-9 models has developed a distinct personality of its own and has become capable of programming itself, as do the newer robots.

In the post-flight lounge, the Robinsons are debriefed. The interviewers are amazed to learn that the Robinsons had encountered intelligent life other than man during their voyage, for no one else ever had. It is the first time that Earth at large learns they are not alone.

Not only does Alpha Control learn of the Robinsons' adventures, but the travelers begin to understand what had happened on Earth while they were away. The colonization program that had begun with the Jupiter 2 mission came to a screeching halt following the apparent failure of the Robinsons' craft. That, combined with the previous failure of the Jupiter 1 mission, had led to Senate hearings and public apathy toward Alpha Control. The United States' space budget was subsequently cut to the bone, allowing only a handful of manned missions to take place in the following years.

The Jupiter 3 mission had successfully reached Alpha Centauri in the year 2007. That triumph allowed Alpha Control to renew its colonization program, although at a much slower rate than that envisioned at the time of the Jupiter 2's launch. Project Lighthouse was launched in 2010, and over the next nineteen years twenty-one more colonization ships, the Victory series, began a two-way shuttle route between Earth and Alpha Centauri. As of the time of the Jupiter 2's arrival on Earth, more than a thousand people, American and European, live at the distant colony.

Deep in outer space, something else homes in on the Project Lighthouse beacon. It heads toward Earth at maximum speed.

The press has a field day. The Robinsons are world heroes, and they find themselves in the spotlight continually. Ticker-tape parades, interviews, and personal appearances fill their days. Don performs the commencement ceremony at the Air Force Academy. Will lectures at universities, stopping off to visit friends at a little town called Hatfield-Four Corners. Penny and Judy tour the country, learning first-hand about the changes that have taken place.

A few months later, Maureen and her children, along with Don, Smith, and Joshua, attend a ceremony honoring her late husband. The John Robinson Space Research Facility opens to scientists from across the world.

Immediately after that dedication, Smith is placed under arrest. From deep within the Robot's memory banks, Alpha Control had learned of his sabotage mission, and they charge him with the crime. Earth, sweet Earth, becomes a prison for him as he is placed in custody, awaiting trial. The Robinsons, having already known of Smith's true reason for being aboard ship at launch, nonetheless implore the government to release him. Alpha Control responds that the charges are not theirs to drop -- his crime was treason against the United States.

Several more months pass. The Robinsons settle down into normal lives, living in base housing near Alpha Control. Penny meets a military pilot named Roger and they begin a relationship, spending most of their time together. Will visits Smith often, wishing there was something he could do to help him.

Alarms at Alpha NORAD sound. An unknown intruder is approaching the planet at high speed. It refuses to answer all requests for identification. A military response is launched as the thing begins to enter the atmosphere.

Earth is about to learn that there is a downside to advertising one's presence to the rest of the galaxy.

Nuclear-tipped tactical missiles impact against the intruder, but have no effect. Power levels all over the planet begin to drop. A strange plague of lethargy instantly sweeps man and animal alike. The immense alien intruder hovers over the cities of the Earth, feeding on all energy, whether mechanically or biologically produced. Oddly, it seems to ignore solar energy, however. Mass evacuations of all cities take place as everyone heads into the rural areas toward what they hope will be safe haven. The lethargy plague intensifies, and as their life energies are drained the people begin dying. In the twelve hours following the alien's arrival, the world is nearly paralyzed. Estimates indicate that man has less than a few days to live, as does Earth.

Penny manages to get through to her mother by phone. She is trapped in Washington, D.C. with Roger, with whom she had been touring the National Air and Space Museum. Through the heavy static, Maureen is able to learn that Penny has taken shelter at the Smithsonian.

Three evacuation ships in Europe and China launch into space. With its restricted space budget, the United States has nothing to launch from its now-abandoned Kennedy Space Center.

Except the Jupiter 2. Only Don and Will know how to fly the thirty-year-old vessel, for it has become something of a technological dinosaur and has nowhere near the automated control systems of Earth's newer, voice-controlled spacecraft. They, along with Judy, Maureen, and Joshua break through the deserted security perimeter at Alpha Control and find the Jupiter 2 hangared, its computer system partially disassembled. Will retrieves the Robot from the computer center, along with stores of old electronic repair parts. Food and life support supplies are quickly gathered. Hurriedly, they manage to refuel, power up the ship and manually fly it out of the huge hangar.

Don has trouble keeping the ship airborne. The alien is draining off the ship's power at an ever-increasing rate, forcing him continually to compensate.

Penny hears a familiar droning sound that gets louder and closer. Dragging Roger outside, they see the Jupiter 2 hovering overhead. It touches down just long enough to pick them up, then struggles back into the sky. Don fears that the ship's power levels are dropping too quickly for them to reach orbit.

A short time later, Smith sits alone in his cell, unaware of the crisis outside. Without warning, there is a great explosion and the wall of his prison cell flies apart to reveal the Robot on the other side. He rescues Smith, and the two of them take off in the Jupiter 2's space pod to rendezvous in space with their mother craft.

The two spacecraft re-dock. Don sets the Jupiter's fusion engine at full throttle and the ship slowly limps out of Earth orbit and into space. Maureen monitors Earth's radio communications as they slowly fade in power. Smith laments the fate of Earth, remembering how he had, for so long, wanted to return there.

Suddenly, all of the radio signals go horribly silent. Behind them, they see the Earth's green continents turn pale and brown as the life is literally drained out of them. The alien intensifies its terrible, mindless act, drawing off the planet's energy even faster as the power diminishes. Nothing can save Earth. As the Robinsons watch, the planet dies.

Power levels onboard the Jupiter 2 increase as the ship slowly puts more and more distance between itself and Earth. All aboard are deeply affected by what they have seen happen to their homeworld, though they are glad to have escaped with their lives.

Their relief is short lived.

The ship's power levels begin to drop again. The Robot reports that the alien intruder has abandoned the lifeless Earth and is now in pursuit of them. Don and Will push the Jupiter's fusion drive for all it will give them, but they realize that they are fighting a losing battle. The intruder will overtake them in mere minutes.

Will comes up with a plan. They head directly toward the Lighthouse beacon, their engines straining to give them more speed. Don knows what Will is trying to do, and helps, knowing that his plan is their only chance. The alien closes quickly. Penny embraces Roger. The Robinsons pray.

The intensity of the beacon's signal reverberates shrilly throughout the Jupiter's hull, which at such close proximity is acting like a tuning crystal. The intolerable sound is deafening. The signal overloads the Jupiter's communications system. The immense, powerful, artificial quasar looms ever larger in the main viewport.

The Jupiter 2 reaches the beacon and swings past it, missing by only a few feet a direct collision with the huge structure. Immediately, Don cuts the power throughout the ship, killing the engines, the lights, and the gravity systems. The ship coasts on, little more than a projectile.

The intruder loses them in the 'blinding' intensity of the beacon's signal. It stops at the beacon, spending a few moments to drain off the Lighthouse's great nuclear power reserves. It hangs there in space as the Jupiter drifts quickly away. The Robinsons dare not breathe, dare not move, dare not speak. Silently, holding their collective breath, the eight refugees from the deceased Earth hope that they can get far enough away that --

It works. The automated alien, having finally drained the beacon, has lost its fix on the energy-silent Jupiter 2 and heads off in another direction, into deep space. The Robot reports that they have reached a safe distance, but Don and Will elect to coast on another few hours before re-activating the fusion system.

Once more, they are in flight. But there is a difference this time -- they know where they are, they are fully-fueled, and they're near the survivors of Earth. What remains of humankind now lives on Alpha Centauri, and Don proposes that they set course for that long-promised planet. All agree that on that world lies their only chance for survival.

Three decades after their original launch, the space family again begins to cross the expanse of cold, milky sea separating Earth from its nearest neighbor in space. To relieve the stresses their ship has suffered, they travel at reduced speed.

During a final meal, Don and the Robinsons lovingly remember their fallen friend, husband, father and grandfather, John Robinson. They all carry him within themselves as they begin again.

After repairing the ship's computer, Don activates the Jupiter's autopilot, locking it securely upon Alpha Centauri. The Robot devises a rotation schedule for using the ship's six freezing tubes, and Don, Judy, their son, Joshua, Penny, Roger and Dr. Smith enter their units and dream of home. Will and the Robot sit down to a game of chess. Maureen, unable to use the freezing tubes due to a heart problem she suffered the first time she did so, opens her husband's old flight journal and makes the first entry it has seen in nearly twenty-five years.