As Troi returns from a class reunion, Dr. Crusher updates her on the situation while serving bridge duty. Deanna gets very curious as to why Bev would take the bridge officer's exam and become a full commander in the first place, and takes Bev's answer of wanting to stretch herself to heart. In the meantime, there is no response from Data, who is away on Barkon Four to retrieve radioactive fragments from a crashed probe.

On Barkon, home to a pre-industrial society, a villager, Garvin, is talking to his daughter Gia, when suddenly both see Data walk into the village, dazed. His clothes and hair appear singed, and when he opens his mouth to speak, only a machinelike humming is heard. Garvin sends Gia home and tries to communicate with Data, who manages to get his voice under control. Garvin quickly finds, however, that Data has great difficulty communicating - at first, he cannot even understand what is said to him, merely mimicking Garvin's statements. Later, his comprehension begins to return, but he has no idea who or where he is, remembering only that he walked to the village from the mountains, a great distance away. Garvin, observing the fragment container Data is holding, asks if he can examine it, and Data agrees. Data finds that he can read the lettering on the container ("RADIOACTIVE"), but has no idea what it means, and speculates that perhaps it is his name. "It's not like any name I've ever heard," replies Garvin, taking and holding up a piece of metal from the case...

Meanwhile, on the Enterprise, Troi informs Riker that she wants to become a full commander and take the bridge officer's test, "to stretch myself." Although she admits seeing old friends at the reunion was a trigger, she says that she's actually been considering the idea ever since her brief stint on the bridge in a time of disaster two years earlier. Riker agrees to support her, but also warns that he's the one who'll have to evaluate her, and not to expect any favors.

On Barkon, Talur, the village's resident scientist, examines Data and pronounces him fit in all respects save his memory loss. (His heartbeat, in particular, is "very regular.") As far as his odd appearance is concerned, she grants that "my grandmother would have called our friend here a demon, or a spirit, or some sort of monster - but current scientific methodology allows us to dismiss such ridiculous superstitions and concentrate on scientific reality." What is he, then? An "iceman", or so she says - from a race that lives in the mountains under very harsh conditions. As Talur leaves, Gia enters and helps Garvin pick a name for Data: "Jayden."

Garvin and "Jayden" leave to find Skoran, the village smith, who might be able to help identify the strange metal Jayden brought with him. Skoran notes that the metal is strangely warm, and that given its quality it must have been tempered in some way. He offers to buy half the lot from Jayden to make jewelry, a prospect to which Jayden agrees. Just then, an anvil falls onto one of the other smiths. He screams in pain, and others run for help - but Data walks over and lifts it off him easily, allowing him to scramble out from under and get medical help. "Did I do something wrong?" he asks Garvin, perplexed by the stares this action has provoked. "No," muses Garvin, "just unexpected."

That evening, Talur assures Jayden that all icemen have such strength, in order to fight off the wild creatures living in the mountains. When Jayden reminds Talur that no one has ever seen such creatures, she scoffs that it's simply a well known fact. Garvin, meanwhile, is beginning to look and feel very fatigued, and Talur takes him out for some fresh air. Jayden talks to Gia, who tells him her mother died a year ago. "Father said she went to a beautiful place, where everything is peaceful, and everyone loves each other, and no one ever gets sick. Do you think there's really a place like that?" Jayden moves to the window and stares up at the stars. "Yes. I do."

Troi, having passed all but one test in the meantime, takes the engineering qualification - and fails, badly, destroying the Enterprise in simulation. Riker reassures her that it's a very hard section, but refuses to tell her what she did wrong, since she'll have to take the test again.

Jayden, meanwhile, sits in on Talur's open-air schoolroom, but objects to her characterization of "rock, fire, sky and water" as the four core elements of the universe as reasoning by analogy. Talur is not impressed, reassuring the children that Jayden's memory lapses are still present, and Jayden is thus not the most reliable of observers himself. The class is dismissed, and Jayden is far from angry - but he is certain Talur's statements are wrong. The issue is soon abandoned, however, when Garvin collapses while arguing with Skoran over money. Jayden and Gia get him home.

At Garvin's home, Talur examines him, but has no idea what's wrong with him. Upon noticing that his lesions resemble burn marks, however, she suspects "the fluids of [his] body have overheated", and recommends lots of water, fresh air, and various herbs to cool the fluids down. After she leaves, Jayden decides (with Garvin's permission) to try to investigate the illness himself, and takes Gia into the village to get supplies. In the village, however, he finds out that the disease is spreading, affecting Skoran and others as well - and that the villagers believe him to be the one responsible for the plague. He and Gia, with supplies, return home.

A while later, Talur finds Jayden examining skin samples from both Garvin and Gia, who has also come down with the illness now. Talur looks through Jayden's magnifying device, far more powerful than her own, and professes some skepticism when Jayden begins discussing cellular damage. She agrees with his ideas of searching for a common experience Garvin, Gia and Skoran have all shared recently, however, and notes that Jayden himself is a likely candidate. He agrees, but also points out that Talur has had extensive contact with him and has not taken ill. Gia comes down to report on Garvin's condition and is sent back upstairs to bed, but not before Jayden notices the metal pendant she wears, and finds that she's had it for a few days, and that it was made out of the metal Skoran bought from Jayden himself...

Troi, back on the Enterprise, is studying once again for the engineering exam and is annoyed at the technobabble. She is far more upset, however, when Riker enters and informs her that he's cancelling the test, saying that his first duty is to the ship and that he cannot let someone serve as a bridge officer who is unqualified. Troi fumes, but then realizes something important in Riker's statements and departs for the holodeck. There, she averts the disaster that has stymied her before, by ordering Geordi to repair a conduit directly despite the radiation in the crawlway - in other words, by ordering Geordi to his death. Riker closes down the simulation and congratulates her on passing the test - the entire point of which was to see if she could make that hard choice.

Jayden, in the village, shows Talur his findings, and tells her of his theory that the metal is sending out invisible particles which are some sort of energy source, and which also are causing the mysterious illness. She is incredulous, but Jayden is insistent - and what's more, he's noticed that the container he brought the metal in seems to block the particles, so he suggests that the container was a safeguard, and the "RADIOACTIVE" label was a warning. He asks her to go to the village and collect all the metal she can into that container, while he continues to search for a cure. Promising to come back and examine Jayden's data in detail, Talur leaves - but shortly thereafter, Skoran comes in with several other villagers, convinced that Jayden is trying to kill them all.

Skoran tries to club Jayden and misses - and doesn't get a second chance, as Jayden neatly sends him against a nearby wall. Skoran's cohort, however, delivers a glancing blow along Jayden's face, and rips off the "skin" concealing Jayden's circuitry. Both villagers stare at Jayden in horror. As Skoran blurts "What are you?" and leaves in a hurry, Jayden puts his hand to his face and replies, "I do not know..."

Later, Talur has collected the metal, but Skoran and the others are back, intent on finding and killing Jayden "before he kills us all." Garvin and Gia do not believe Jayden can be such a monster, but cannot stop the others from departing on their mission. Talur tells father and daughter to rest, and leaves herself. Gia wanders through the house, but stops when she reaches the kitchen and hears a voice calling to her from the corridor. "Jayden!"

Jayden has returned, hooded, and needs time to find a cure. Gia tells him to remove the hood so she can see him, and gasps in horror when he does so, but stands her ground, willing to help. After hours of work, he finds this cure and, having already administered it to Garvin, gets Gia to drink it. He reasons that the villagers will not take it voluntarily, and decides to spike the village well with the solution to help them.

Jayden arrives at the well and prepares to drop in the cure, but Skoran and the others catch him in the act and accuse him of causing the plague and attempting to kill them. Hastily, he drops the compound into the well, just as Skoran impales him with a sharp metal pole. Skoran is stunned by the electric shock, but "Jayden" is worse off, falling down dead in a heap.

Much later, a fully recovered Gia is walking in the village when Beverly and Riker ask her gently if she's seen a friend of theirs. She says she has seen him, but points to a grave when they ask where he is.

"They killed him because they were afraid of him, but he saved all of us from the sickness." She describes his work over the last few days, and tells them the metal fragments are buried in the forest. "What was his real name?"

"Data."

"Data ... he was my friend too." Gia hurries off.

Riker and Bev quickly scan and confirm that it is Data, and prepare to beam both him and the metal up in secret.

Data is successfully revived, but with no memory of his experiences after a power surge overloaded him as he was working with the crashed probe. He examines his clothes, however, and concludes, "It appears I had an interesting time." As Riker and Bev fill him in on what little they know, Deanna departs for her bridge shift, telling Data that because of her promotion, "you can call me `sir' from now on."