- The Control Voice: The Book of Job tells us that, "He who discovereth deep things out of darkness, bringeth out to light the shadow of death". Will man's unquenching thirst for discovery, ultimately be his undoing?
The Mars III manned expedition to Mars is in its 315th and final day on the surface when the crew discover a cave containing strange alien writing and a capsule. The capsule suddenly opens and the crew is knocked unconscious.
The crew, Pete Claridge, Ed Barkley and Alan Wells, wake and find they have been unconscious for an hour, and have only four hours left before they must lift off and begin the trip back to Earth.
Day 525, and the Mars III spacecraft is almost back to Earth. An explosion rocks the craft and it begins to veer off course. After correcting its course, the crew discover that half of their oxygen is gone, they have no communications with Earth and Ed, the engineer, had suffered a head injury.
They find a gelatinous substance attached to the wall of the control room. Upon examining it, they discover that it is living matter. It is unlike anything previously known and they speculate that it may have come from Mars, possibly even the capsule they found. Further failures occur on the ship, including life-support. Ed investigates and manages to patch the system back together.
Alan, one of the crew, begins to behave strangely and Pete suggests taking a blood sample to check. Ed arrives back and finds Alan alone (Pete is running some tests) trying to hide the fact that his arm is covered in the gelatinous substance they found on the wall earlier. Alan transforms into an alien creature and attacks Ed. Ed drives the creature into the airlock, with the intention of venting it into space. It transforms back into Alan just in time for Pete to arrive and see Ed open the airlock. Ed tries to explain what happened to a skeptical Pete.
Having seen the creature infect and take over Alan, Ed does not believe that Pete can be trusted. He goes to fix some other systems and finds Pete's dead body in a spacesuit, confirming that the man in the control room is not the real Pete Claridge.
The alien creature explains that the death of his crewmates was unavoidable and an unintended cost of his lifecycle, but that particular phase has passed. A further serious failure occurs on the ship, and only Ed can fix it. The alien pleads with Ed to help him, and explains that it can bring many benefits to Earth cure for cancers and environmental technologies. Still skeptical but hoping there may be a way out, Ed works with the creature to fix the problem.
Communication with Earth is restored and "Pete" talks to his family. Ed looks as though he is about to tell Earth about the alien creature when it mutes the communication and explains that if he does, they will not allow the ship to land, leaving him stranded, and with no life support he will not survive.
Just as the ship begins its descent, Ed fires the engines and sends the ship wildly off course, plunging too quickly into the atmosphere. He believes that the creature wants to get to Earth at all cost and begin reproducing by taking over the population. The alien creature, with all pretense dropped, changes into its natural form and tries to take over Ed. Ed opens the airlock, killing himself and the creature. The ship is completely destroyed as it breaks up and burns in the atmosphere.
- The Control Voice: The true measure of a hero is when a man lays down his life with the knowledge that those he saves... will never know.
Not to be confused with Star Trek IV: The Voyage Home.
The Tropes Home:
- Alien Blood: This episode's alien's blood is green and slimy.
- Bizarre Alien Reproduction: The alien that invades the Mars III spacecraft reproduces by releasing a spore into an organism's body which then leaves the body and replicates it. The process is fatal to the organism in question. The alien's plan was to proliferate its species by converting humanity en masse once the spacecraft returned to Earth.
- Cure for Cancer: The alien in the form of Pete Claridge tells Ed Barkley that it will give humanity the cure for cancer once it arrives on Earth.
- Family Man: Pete Claridge is a dedicated family man, to the point that he mildly irritates his crewmates Ed Barkley and Alan Wells with how much he talks about his wife Jenny and daughter Laura during the Mars III expedition.
- The Greatest Story Never Told: This episode revolves around a trio of astronauts traveling back to Earth from Mars. Earlier, two of the astronauts had been replaced by aliens, leaving just the one human who eventually learns about the impostor. Forced to choose between making it back to Earth and the fame and glory he would receive and preventing the alien species from spreading to Earth, the final astronaut finally decides to be a hero and sabotages the re-entry procedure causing the ship to burn up, with Ground Control believing it to be a disastrous malfunction. The ending narration: "The true measure of a hero is when a man lays down his life with the knowledge that those he saves... will never know." However, in "The Voice of Reason", it was later revealed that the black box from the ship was recovered. In that episode, Randall Strong played the recording for the Committee that was charged with investigating alleged alien activity which could threaten the United States.
- Heroic Sacrifice: The last human member of a three-man space expedition returning from Mars blows up the ship to stop a hostile alien from reaching and infecting the Earth.
- The Right of a Superior Species: The alien in the form of Peter Claridge intends to proliferate its species on Earth at the expense of humanity, saying, "Our species is millions of years old. It is our right to take lives in order to continue."
- Through the Eyes of Madness: Played with. The three-man crew of a spaceship are slowly going mad after returning from a mission on Mars. At one point the pilot suddenly transforms into an alien creature in front of the engineer, who jettisons him into outer space. Except when Peter Claridge shows up when this is going on, the 'alien' one looks completely normal and begs him to stop their insane colleague. It turns out that they were both aliens who had assumed their shapes, and the engineer was the last real human on board.
- Thrown Out the Airlock: An astronaut jettisons one of his crew members because he thinks that the guy turned into a monstrous alien in front of him.
- What You Are in the Dark: The closing narration says, "The true measure of a hero is when a man lays down his life with the knowledge that those he saves... Will never know."
- You Are Who You Eat: There's a shape shifting alien which assumes the form of the people it eats.