The series finale of Star Trek: Reliant aired on March 3, 1981. The episode starts on the Reliant bridge. Here are the important scenes, so this is only part of the episode.
Terrell: So we don't have a candidate planet yet. We've explored about fifty planets so far, and nothing works. This is getting frustrating.
Chekov: There is Ceti Alpha V, about 5 million kilometers away.
Terrell: Let's check out that planet. Xon, begin scanning for life forms.
Xon: Scanning. Life forms indeterminate due to atmospheric interference. Nitrogen and sulfur dioxide atmosphere, with some traces of oxygen, carbon dioxide, and water vapor
Chekov: Well that wouldn't support much, if any life.
Sulu: I don't think it's a good idea to even consider that planet.
Terrell: Why, Mr. Sulu.
Sulu: It's a very long story. It goes back to my first mission on the Enterprise.
STAR TREK RELIANT INTRO AND THEME
Sulu: When I was on the Enterprise, we scouted the Ceti Alpha system, almost fifteen years ago. Captain Kirk at the time left Khan Noonien Singh and his followers, along with another member of the Enterprise crew, Lieutenant McGivers, on the planet.
Terrell: What happened to Ceti Alpha V? Kirk would not have left anyone on a lifeless planet.
Sulu: The planet wasn't lifeless when the Enterprise explored it. A little rough around the edges, but certainly not lifeless. That planet can barely support any kind of life with the atmosphere it currently has.
Terrell: That doesn't get us from then to now. Xon, Chekov, I want some answers. What happened to Ceti Alpha V?
Xon: There is a lot of planetary debris near the projected orbit of where Ceti Alpha VI should be, Captain.
Chekov: Ceti Alpha VI must have exploded, and that affected Ceti Alpha V's atmosphere.
Xon: It is likely that the destruction of Ceti Alpha VI altered the orbit of Ceti Alpha V, changing the composition of the atmosphere from a livable planet for humanoids to an inhospitable environment, Captain.
Terrell: But there's one thing I don't understand. How could Ceti Alpha VI simply explode and Federation scanners not pick up the debris?
Sulu: As I recall, Ceti Alpha VI was about five times larger than Ceti Alpha V, and exerted a small gravitational pull on the planet. This star system is also near L-370, which was completely destroyed by the Doomsday Machine planet killer.
Terrell: So it's possible that the Doomsday Machine destroyed Ceti Alpha VI?
Sulu: Anything's possible.
Terrell: That wouldn't explain why the Doomsday Machine did not destroy Ceti Alpha V as well. As I recall, that weapon was an indiscriminate planet killer.
Chekov: Maybe it wasn't indiscriminate. Ceti Alpha V is a small planet, about three-quarters the size of Earth.
Xon: I disagree with Commander Sulu's hypothesis. I believe that an unstable inner core exploded and destroyed Ceti Alpha VI.
Sulu: That could be true. Commander Spock detected unusual planetary core readings from Ceti Alpha VI when we were exploring the system. However, Spock determined that destruction of that planet was unlikely.
Terrell: But how could it have happened without the Federation knowing about it?
Sulu: Captain Kirk asked Starfleet Command to set aside this star system as unclaimed territory, not to be disturbed for at least fifteen years, to determine how Khan's people progressed.
Terrell: Well they didn't progress far. They didn't have a chance to, based on what happened. Xon, scan the planet again for life signs.
Xon: Scanning. Life signs indeterminate. Too much atmospheric interference.
Terrell: So we don't know what's down there.
Sulu: Aside from the ruins of whatever settlement Khan established, there's likely nothing down there.
Chekov: Nothing could possibly have survived down there.
Terrell: So we might finally have a candidate for the Genesis Device. Lieutenant Garvin, get me Dr. Marcus at Regula I.
Garvin: Subspace channel established, Captain.
Terrell: Dr. Marcus, this is Captain Terrell on Reliant. We might finally have a planet for you.
Carol Marcus: Well that's delightful. Where are you right now?
Terrell: Ceti Alpha system, in the far reaches of Federation space. Ceti Alpha V is the planet. Not too close or too far from its sun, appears lifeless. The planet also has an atmosphere, which apparently assists with your research.
Carol Marcus: Well that's a long way from Regula I or most of explored space.
Terrell: We believe Ceti Alpha V is lifeless, but our science officer is unable to fully determine whether life exists on the planet. There's too much atmospheric interference.
Carol Marcus: Well you'll have to send a landing party down there to take further scans.
Terrell: Out of the question. I will not risk any members of my crew in those inhospitable conditions.
Carol Marcus: If Reliant fails to find a suitable planet, Starfleet Command will ensure you never sit in a captain's chair again, Terrell. Marcus out.
Terrell: So who wants to go down there? I'm opposed to this, but I apparently don't have a choice in the matter.
Sulu: I'll go. I'm curious as to what happened to Khan.
Chekov: I'll go as well, but I don't think we'll find anything. How could anyone or anything have survived down there?
Terrell: No fishing expeditions, Commander Sulu. The moment you find anything that is remotely life like, you are to return to Reliant. Understood?
Sulu: Aye, sir.
In the next scene, Sulu and Chekov are in life support suits, heading towards the transporter beam.
Arias: Transporter ready, Commander Sulu.
Sulu: Energize. (Sulu and Chekov beam down to Ceti Alpha V).
Chekov (almost knocked down by the wind): Tricorder scanning, Sulu. No life forms detected. There's nothing here.
Sulu: Keep on scanning, Chekov.
Chekov: Aye sir.
Sulu: We're about three miles away from where the Botany Bay, Khan's ship, originally landed.
Chekov: Captain Terrell said no fishing expeditions, Sulu.
Sulu: But we have to find out whether there's any life in that space ship. If there is, we report it right away to Reliant, they beam us back, and we tell Dr. Marcus that this is not a suitable planet.
Chekov: Understood, but the only thing I think we'll find is a shipwreck.
Sulu: Not unless we actually go to the ship and find out there's no life present.
Chekov: Yes, sir.
Chekov looks ahead. In the distance, something that looks like an apparition, in human form, appears.
Chekov: I see something over there! A human life form, perhaps.
Sulu: That's impossible. Even Khan couldn't survive in this atmosphere.
Chekov: If I remember my history correctly, Khan was a genetically enhanced tyrant. He could have survived, even in these conditions.
Sulu: Scan over there, Chekov.
Chekov: Yes, sir. Picking up what appears to be a life form, but the tricorder is saying indeterminate reading.
Sulu: I'm going to believe you, Chekov. If that is a person, then we know this planet supports life, and we can't use this planet to test the device. Sulu to Reliant.
Terrell: Reliant here.
Sulu: Captain, we believe that we've found a human life form down on the planet, but we cannot determine exactly whether it is alive. Tricorder readings are indeterminate.
Terrell: Are there any other life forms aside from what you believe you saw?
Sulu: Negative, sir.
Terrell: This is damned peculiar. I'm going to bring you guys back up here.
Sulu: I see a ship ahead.
Terrell: Do not approach it. Does the ship have any markings on it?
Sulu: I'm having difficulty seeing through the atmosphere and life support suit. I can't make out anything. If it is the Botany Bay, its surface was scoured by the atmosphere.
Terrell: I'm bringing you both back up here. Doctor Marcus wants Commander Chekov to pilot a shuttlecraft to Regula I and brief her on our findings. We are going to remain in orbit around Ceti Alpha V.
Sulu: Yes, sir. Sulu to transporter room. Two to beam up.
Sulu and Chekov beam back. About 5 seconds later, several ragged-looking men appear from the shadows on Ceti Alpha V, in masks.
Back on the Reliant, Terrell debriefs Sulu and Chekov.
Terrell: So what happened down there?
Sulu: We saw a ship, which was probably Khan's old ship.
Chekov: I also saw what appeared to be a human figure.
Terrell: How could humans survive in a sulfur dioxide atmosphere?
Chekov: It should be impossible. However, Khan's people were genetically enhanced humans, and it's possible to believe they were ingenious enough, or strong enough, to survive there.
Terrell; You realize, if I report this finding back to Dr. Marcus, that we'll be on a wild goose chase searching for life on that barren planet, or for other planets?
Sulu: I think we should investigate this further. There are scientific benefits in finding out what the destruction of Ceti Alpha VI did to Ceti Alpha V.
Terrell: I'm inclined to disagree. This is extremely odd. I'd rather search for another planet.
Garvin: Captain Terrell.
Terrell: Terrell here.
Garvin: Dr. Marcus wants another report on Ceti Alpha V.
Terrell: Tell Dr. Marcus I'll send Commander Chekov in a warp-capable shuttlecraft to Regula I with all our findings.
Garvin: Yes, sir.
Terrell: Chekov, so you're going to Regula with all the data. Sulu, if we go back down to that planet, I'll accompany you.
Chekov: Captain, I think this Genesis device research is a disaster in the making.
Terrell: I agree but that's not our mission or our ethical debate. Go to the shuttlecraft with the test data and report to Dr. Marcus on Regula I.
Chekov: Aye, Captain.
Sulu: If there were humans down on the planet, I recommend that we go down there armed the next time. It's almost certain that whoever or whatever is down there won't have phasers or other advanced weaponry.
Terrell: That is exactly what I was thinking, Sulu. Let's return to the bridge.
Sulu: Aye, sir.
On Ceti Alpha V, Joachim, Khan's son, talks with Otto, an older member of Khan's party.
Joachim: We saw two men in spacesuits. We should report this back to Khan.
Otto: Somebody was looking for us. Maybe it was the same people who left Khan and the rest of us here all those years ago.
Joachim: Khan must know. Maybe Kirk was one of the men in the spacesuits.
Otto: I do not believe it was Kirk. I think it was men under his command.
A voice emanates from the Botany Bay. It is Khan.
Khan (voice): What did you see out there?
Joachim: Two men in spacesuits.
Khan: Captain Kirk?
Joachim: We could not get close enough to see them.
Khan: Kirk did not forget about me after all...