McCauley and Lt. John Leonard are at a radio observatory on the far side of the Moon, taking care of the third member of their group: Major Charlie Randolph. John blames himself for Charlie's injury and wonders how they can get Charlie back to Earth in his condition. McCauley tells him to focus on his work and John uses the radio telescope to scan distant stars for signals. Charlie moans in his sleep and starts to wake up, but then passes out again. John asks McCauley for permission to finish the colonel's tape report and get the whole story on tape, and McCauley agrees.
John and his fiancée, Donna Talbot, are attending a dinner party at the McCauley house. Charlie is running late and the major's girlfriends keep calling to say goodbye before he heads out on the expedition. The major arrives, carrying roses and a guitar, and has lipstick on his forehead. He complains about how he's all alone unlike his two friends and their wives, and McCauley points out the lipsticks. Charlie claims that it belongs to his niece just as another woman calls for him.
As they wait for Charlie to finish the call, Donna asks about their mission. McCauley explains that they're going to be monitoring radio waves from distant stars. The observatory is on the far side of the Moon so that they can avoid interference from Earth signals. Charlie finishes his call and they go to dinner.
The next day, the three men arrive on the Moon and proceed to the observatory. As they wait for the camera to focus, Charlie suggests that they record a mock radio broadcast to send back to their friends on Earth. McCauley serves as the announcer and John describes the view outside the observatory. Charlie finally sings an improvised song about the Moon and love.
Once the camera is online, the three men start recording the signals of hydrogen atoms in space. The lights flicker and they confirm that the fault is at the radio telescope. The trio head to the telescope and discover that the drive unit is jammed. Once Charlie finishes repairs, they head back to the observatory, trailing the auxiliary power cable behind them. McCauley warns John that the insulation is brittle because of the extreme cold. As they head back, John snags the cable on a rock. When Charlie goes back to free the cable, it snaps, electrocuting him. The shock knocks him off a nearby cliff and he falls to the lunar surface several dozen feet below.
Charlie hasn't woken up since the accident and John blames himself. McCauley points out that they can't treat their friend and need to get him back to Earth. However, John warns that if Charlie suffered any brain damage then the g-forces could cripple or kill him. McCauley notes that they have no choice and tells John to pack for departure while he goes to start the spaceship.
At the rocket, McCauley finishes preparations and calls John. He's surprised to hear Charlie singing on the radio and heads back to the observatory. John is playing the recording they made earlier, and tells McCauley that Charlie woke up when he heard it. Charlie is now conscious, but in heavy shock. He insists on hearing the sound from the stars, and McCauley has John turn on the radio telescope. Charlie, dazed, says that it's a good sound and lays back.
John monitors the transmissions and informs McCauley that he's picking up a repetitive wave pattern. They confirm that it's coming from a distant star, Cetis, over 200 light years away. The two men realizes that the wave pattern forms a specific mathematical pattern, and McCauley has John record it. The colonel then transmits a broadcast, identifying themselves and playing Charlie's song.
McCauley tells John that they'll wait for a response, and Charlie wakes up and tells them that it will take 400 years for the signal to go back and for them to receive a response. They point out that Charlie was the one who wanted to send the song, but he has no memory of what happened. When McCauley says that they're taking him back to Earth for treatment, Charlie insists that he's fine and jokingly says that now he has an excuse for them to wait on him.
Once the mission is over, the McCauleys have another dinner with John, Donna, and Charlie. Charlie is busy on the phone with another of his many girlfriends. Once he hangs up, McCauley asks how he'll handle it if there's an alien girl on Cetis, waiting to hear from him and then send a message back. The phone rings again for Charlie and the major says that he'll figure out a way to get by until then.