Rod Serling wrote a two-part script for episodes one and two of a Planet of the Apes TV series. The earliest dated TV script was "A Fallen God", from March 2, 1973, and the production codes on the scripts seem to suggest this was written soon after Serling's script - both follow a 'series concept' outline featuring Virdon, Kovak and Galen. Serling's script was a peculiar mixture of the TV concept, a very early script written by him for the original movie, and the plot of the second movie. It is significantly different to the filmed episode, although it does set up the series scenario in line with the concept, and some of the ideas were carried over into Art Wallace's script for Escape from Tomorrow. The plot concerned Virdon and Kovak landing on a mysterious planet after a period of suspended animation (Kovak: "To age four weeks...while travelling almost six years...and to know that nothing we've left behind will be the same when we return...if we return"). The reason - to rescue or find a crew of missing astronauts (Virdon: "Somewhere - out there - are Taylor, Thomas, LaFever and Bengsten - or their remains. We're going to find them...or finish an epitaph for them." Here, the story begins to mimic that of Beneath the Planet of the Apes, while the names recall earlier ideas of Serling's: George Taylor is, of course from the first movie; John Thomas was the name of the central character in Serling's early draft script, subsequently changed to Taylor; Paul LaFever was also in the early script, the character later became John Landon - the other astronauts on that mission were Dodge and the deceased Blake). The continuity of this story is somewhat confusing, it's almost a sequel to his first script, which was revised heavily for the movie, rather than tying-in with any of the actual movies.
They have landed near a beach, and there, they find fragments of metal which they recognise as parts of Taylor's ship.
Kovak: "Then where are we?"
Virdon: "Roughly where we thought we'd be. In the Constellation Casseiopeia. Right on the border of the Milky Way"
Kovak: "If they survived this crash...they sure as hell could have survived this air."
That night Virdon explores the area and discovers "four makeshift wooden crosses. From one of them hangs an astronaut's helmet...the worn, weathered, almost obliterated name on the helmet...reads 'Taylor'." They exhume the bodies: (1) male, caucasian, age approximately 31, [gunshot] most likely; (2) male, caucasian, age 28 to 30, same cause of death; (3) male, caucasian, age approximately 23, broken neck; (4) female.
Virdon: "There were no females on that crew."
They are the bones of an ape.
Virdon: "I'd like to find that other crew member - whichever one he is. God knows, I wouldn't want to leave him here alive." An clothed ape is watching them, Kovak shoots him in the neck. They bandage the injured ape and go to search for the remaining crew-member. On their return they discover the ship has been ransacked. As they are about to kill the ape, he suddenly speaks -
"My name is Galen." He tells them, "The ship arrived with four 'men'. When that first ship landed from the stars, the four men who stepped out threatened to change who was the hunter and who was the prey. They were killed by our Security Police."
Virdon: "There are three human skeletons down there, the fourth is an ape."
Galen: "Her name was Zira. She was helping them to escape. As to the other astronaut, best you forget him."
Galen: "He's dead."
Captured while trying to find tools and spare parts in the ape city to repair their damaged ship, they are taken before the ape assembly.
Mr Zendee (an ape): "They represent a higher plane of species - not unlike the four individuals who came into our midst some time ago - whose deaths occurred without the sanction of this body."
Ursus: "Whose deaths, I may interject, occurred while the animals were violently resisting all attempts on our part to place them in custody."
Zaius: "You knew...that these space travellers offer a link to our own history." "(They) have moved ahead in time, as did Commander Taylor and his crew."
At this point, Zaius organises their release on the condition that they leave and return to their own time. He is shot by Ursus and his gorillas as the other three escape. They travel to a Forbidden Zone and find they are in New York.
The second part of Serling's episode is largely taken up with a trial, mirroring the events of the first movie. Renegade humans want to execute Galen and Zonda, the fifteen-year-old, strong but simple son of Ursus, who has been captured while trying to kill the fugitives. Virdon and Kovak reason with the humans until they are all allowed to go free, having all earned a grudging respect for each other (this aspect of the story is reminiscent of the 'Malibu Graphics' character, Grunt, the giant son of gorilla General Ollo, who sides with the humans against his father).
While Serling's contribution hasn't been acknowledged (unlike his work on the movie script), the concept of a previous astronaut crew survived into Wallace's script (Urko: "Another ship, Zaius?! It's hard to believe." Zaius: "More than ten years ago, another such ship landed. Humans. They said they were from Earth...but from another time period...long ago." Urko: "But the danger was eliminated. They were killed." Zaius: "Before they could be questioned."). This plot device isn't explored further in the series, although it might have been had the series continued. 
The short story Who is this man? What sort of devil is he?, written by Robert Greenberger for the anthology Planet of the Apes: Tales from the Forbidden Zone, is a direct sequel to the TV episode 'The Trap' and incorporates Serling's concept into the officially-licensed continuity of the TV series. In it, Urko tells his wife Elta and son Urso about his encounter with the earlier astronaut crew, and his having killed Bengsten, LaFever, Thomas and - accidentally - Zira. The fourth astronaut is here named 'Charles' and he escaped but is presumed dead.