The film starts with a news program. Operation Ganymed is officially abandoned. Three spaceships with seven men crew each had started two years ago to explore the Jupiter-moon Ganymed. The mission was a collaboration between NASA, ESA and the Russian space agency, under the sponsorship of the UN, meant to be a peace-mission and a scientific expedition. There had been no signal from the expedition for half a year and all hope of a return has been given up.
In the next scene we are inside on one of the spaceships. They are approaching earth after almost five years in space, for most of the time, they had no contact to earth. This is the only ship left and there are five crew-members: Mac, the commander, Steve and Doug from NASA, Don from ESA and the Russian cosmonaut Ossorkin (Jürgen Prochnow). Apparently, the other two ships were lost early on, because the Ganymed atmosphere was quite different from what they expected. The astronauts suspect, that some people on earth even knew beforehand but let them start anyway. They are supposed to rendezvous with a shuttle in orbit, which will then take them down to earth.
Don is the only one, who shows his excitement about being back, he asks the commander to turn the ship round a bit, so that they would be able to see earth through one of the portholes. The commander is busy getting the ship ready for the rendezvous and refuses. The quarrel that starts is part of a long series of discussions, which continue throughout the film. Don is the only scientist on board, the other ones all have some army background, so their ideas about the importance of discipline differs considerably. In the end, Mac gives in and it turns out, that they are all overjoyed to be back and keen on seeing the earth.
In preparation for docking to the shuttle, they have to get rid of the part of the ship, that contains the nuclear power source. It is worrying though, that there still is no signal from earth. But Mac insists to go through with the manoeuvre, following his instructions. But the shuttle is nowhere in sight and they only have oxygen for a few hours without the nuclear power. Desperately they try again to call earth, without success.
In the end Mac decides, to go down to earth in a small part of the ship with the help of a software, that was meant to take the scientific samples and the equipment down in the freight compartment. Don dives quickly back into this part of the ship, which they will leave behind in order to get the first-aid and survival kits.
They land on a hostile, deserted shore. It is soon clear, that water will be their main problem. Most of the crew are for staying put to wait for rescue, Mac on the other hand wants to march north, anything but sit still and give slowly up. Mac goes off on his own and after a few hours faints from heat and thirst. Some of the crew have followed him though and save him. They agree to go north but stick to the shore, where they can at least gain some drinking water by evaporation from the sea. But it is not enough for five people, even if they rest by day and walk by night.
The five men find a village on the coast, which turns out to be deserted. But they find a newspaper in Spanish and from a map in a schoolroom, they deduce, that they are in Baja California. While searching for water, Ossorkin and Mike, one of his American colleagues, rest in the shade in one of the buildings and begin a conversation. Doug is rather taken aback, that Ossorkin doesn't want to stay in the US, but to go home to Russia, to his friends and family. They start to speculate, what has happened, maybe the peace-mission didn't help and there was a war after all. And Ossorkin suggests, that their whole mission was nothing but a gesture, that there never was a real scientific purpose to it, that their whole mission had been accomplished after the start, giving people a common, glorious aim and politicians opportunity for wonderful talks.
Their thoughts are interrupted by Don, who shouts, that he has found water in an old cistern. Everybody runs to join him but the water looks rather dubious. Mac suggests, that they should put some disinfectant tablets in, as they are not used to germs anymore. Don hugs the first-aid box to his chest and says, that there are no tablets in it. When the others grab the pack, it turns out, that it contains samples from Ganymed, which Don has taken instead of the first-aid material. For Don, only these samples can justify the suffering of the last years and all the people, who died. The others in their rage force Don to try the water and he throws up.
They now know, where they are and decide to go on until they meet the highway to San Diego, where they hope to meet people at last. They all suffer from thirst and Don has nightmares: he remembers the time on Ganymed. After one more discussion with Mac, he and two colleagues had been allowed to explore a lake. It is very difficult to take samples from the lake and Don's companions die. But the sample definitely contains signs of life, even chlorophyll!
When they finally reach the road, they find it overblown with sand; an aeroplane they see is only a wreck. Despair spreads although Mac still talks about more dense population further north. Ossorkin quietly utters his suspicion, that their skin lesions are not caused by sunburn, as they have only marched by night and their skin has been getting worse as they went north. One of them tries to suggest, that maybe a nuclear power-plant blew up but Ossorkin points out another reason for radioactive fallout: a war between Russia and the US. Doug takes up this idea and goes berserk, he blames Ossorkin for the destruction of his country and kills him. Mac shoots Doug with the signal-ammunition. Steve goes mad, runs away from the others and soon dies in the desert.
There are only Mac and Don left now and it is Don, who keeps them going and holds Mac, when he dies. Don starts hallucinating in the heat, he hears the applause on his return, his reception with a press-conference. He tells the journalists, that life is everywhere, that it is not unusual at all. He tries to answer all the silly questions until he wakes up again in the desert. He stumbles on and in the end reaches a shanty town, built at the outskirts of the deserted San Diego. He is hardly noticed by the people, as they look just as tatty as he does.