A LuthorCorp experiment is released into the atmosphere - a toxin that causes its victims to hallucinate their worst fear. Chloe, Lana, Jonathan, Martha, and Jason all fall victim to the toxin, Clark and Lex, desperate to find a cure, takes a dangerous antidote. Meanwhile, Lionel is released from prison and his conviction overturned thanks to a mysterious benefactor.
→ see also Category:Screencaps from episode 4x10
Lana shows up at Clark's loft wearing a red dress and heels and asks Clark for help because she believes Jason is lying to her. She then admits she shouldn't have given up on the two of them and they start making out. Jason then barges in and beats up Clark, knocking him out. Lana stabs Jason... and it is revealed that Jason is dreaming as he falls off the balcony at the Talon.
Jason is in a coma at the hospital and is deteriorating quickly. Chloe and Clark do some digging and discover that Jason had a job interview at LuthorCorp that morning. They go to the plant to investigate and split up. Clark runs into Lex. He questions what Jason was doing that morning. Lex is high-strung and short-tempered. He tells Clark that he himself didn't meet with Jason, declaring that he's doing everything he can to help him. He orders a security guard to escort Clark out of the building and storms off.
Chloe is walking down a dark corridor when there is a sudden power failure. She finds a spooky basement room with a woman sitting in the middle, her back to the door, wearing a straitjacket. The woman cryptically warns that Chloe can't escape – none of the women in their family have – and tells Chloe that she is her mother; Chloe touches the woman's shoulder and is horrified to see a nightmarish version of herself bound in a straitjacket, and begins screaming. It becomes clear Chloe is hallucinating as Clark and the security guard find her. Clark knocks out the guard and takes Chloe to the hospital.
At a lab within the facility, the reason for Lex's irritability is revealed. He is meeting with a team that was working on an experiment with meteor rocks and unconscious subjects. A bacteria became airborne and several of the workers are unconscious. Lex orders the project (contracted with the military) dismantled, even after the lead doctor, Dr. Ford, points out that the project is worth millions of dollars. Lex says it isn't worth people's deaths and tells him that Jason is infected as well. Dr. Ford confirms the bacteria is not contagious, it is only transmitted though inhalation. He is working on an antidote but it won't be ready for another five hours.
Clark tells his parents what he learned and Lana arrives at their house. She is upset and says she can't sit around watching her loved ones die.
At the prison, the warden reveals that Lionel's conviction has been overturned by a mysterious person with powerful influence and that he can go free. He chooses to remain in the prison as it's where he can "do the most good", but the warden says the prison is not a place where he could come and go as he pleases.
Checking weather patterns, Clark and Lana find the presence of a mysterious "cloud," originating over the lab and seeming to spread. As soon as Clark leaves, Lana is hallucinating that she is in a room with the dead bodies of her mother and father and her friends, and herself as a corpse warning that everyone will leave her. Lana starts screaming and Clark catches her as she faints. Dr. Ford comes up with an antidote but it contains elements of the bacteria. Lex takes responsibility and goes in to administer the antidote. It kills a patient but not before he indirectly rips Lex's suit, exposing him.
Lex goes to the hospital and finds out how far the infection has spread and reveals the nature of the disease. Clark arrives with an unconscious Lana. Then a series of explosions rock the hospital. Clark looks out the window as another meteor shower rains onto Smallville. A meteor bombards him with kryptonite. He tells Lana that he's from another planet and she is appalled. Disgusted by his true origins, Lana "kills" him with a shard of kryptonite... when Clark wakes back into reality, suddenly cured. Clark hurries home to find his parents have collapsed.
Clark then confronts Lex. Clark believes his blood may be the cure since he is the only survivor so far, but Dr. Ford points out that testing for the factors responsible will take too long. Lex states that their only hope is the new antidote, which is yet to heat to the proper temperature. Dr. Ford argues that the antidote is potentially lethal and needs to be tested before it's given to any victims. Clark secretly superheats the antidote while they are arguing, and the antidote is ready for testing hours ahead of schedule. Rather than risk testing it on someone else, Lex takes the serum himself. Lex begins hallucinating a future vision of himself as President launching nuclear missiles and destroying the world. However, the antidote is successful and he snaps out of it.
When Lionel leaves the prison, his mysterious benefactor is waiting in the car for him, revealed to be an old friend (who at that point remains anonymous to the viewer). Lionel laughs and says he should have known.
With the toxin under control and everyone treated, Clark and his parents return to the farm and discuss things. He says that his hallucination has made him afraid to tell any of his close friends about his secret.
Jason is considering leaving Smallville since he doesn't like what he's becoming. Lana senses he is not telling her the full story, but Jason does not come clean.
Clark finds Chloe at the Torch office, very angry that the whole thing is being swept under the rug. He asks why she is so worked up and she admits her anxiety over her worse fear. She admits that she found her mother three months ago, in a mental institution with a hereditary psychiatric disease. She starts to cry and Clark tries to assure her that she isn't necessarily going to develop it too. Chloe stresses that she feels much less scared of having shared her secret with a friend. Because of dream-Lana's reaction, Clark is scared to reveal his secret to anyone. He is forced to renege on telling his own fear and does not take the opportunity to tell her his secret.
The toxin caused many people to have hallucinations of their worst fears (indicated in the recap by the green text). Only a few characters' were actually shown, but Jonathan mentions that his and Martha's fear involved Clark being experimented on.
- Clark Kent - Clark had a hallucination of a second meteor shower. In it, meteors started destroying everything, and one hit directly near him, causing kryptonite to shatter and make him weak. Lana was lying in a bed near him and the meteors made her figure out that it was Clark that caused the first shower and killed her parents. Knowing this, she picks up a sharp meteor shard and stabs Clark with it.
- Lana Lang - Lana appears in a room lying covered on a bed. She realizes that she is not alone there as she sees many other covered beds. As she moves towards them the covers fall, showing her mother, father, Chloe, and Jason dead. Another bed then appears containing her own corpse that gets up and tells her that "they're all going to leave you, and there's nothing you can do about it."
- Lex Luthor - Lex finds himself in the White House as the President. He then goes outside where he sees nuclear missiles flying across the skies and destroying every living thing. Finally, he sees himself in a field filled with the bones of all the people he killed.
- Chloe Sullivan - Chloe walks into a room in which she sees a person sitting in a chair wearing a straitjacket. She approaches the person, thinking it is her mother, but it is revealed to be a nightmarish version of herself with pitch black eyes and a scarred mouth, who tells Chloe that she will have the same fate. The meaning of this warning is clarified at the end of the episode when Chloe reveals that she has discovered her mother is actually in a mental institution and that her disease may be hereditary.
- Jason Teague - Jason dreams about Lana discovering the true reason he came to Smallville. Then she starts making out with Clark, claiming that it was a mistake that they broke up. Jason then comes in and starts beating up Clark for stealing Lana from him. After seeing this, Lana takes a pair of scissors and stabs him, causing him to fall from the Kents' loft.
- Tom Welling as Clark Kent
- Kristin Kreuk as Lana Lang
- Michael Rosenbaum as Lex Luthor
- Jensen Ackles as Jason Teague
- Allison Mack as Chloe Sullivan
- John Glover as Lionel Luthor
- Annette O'Toole as Martha Kent
- John Schneider as Jonathan Kent
- Malcolm Stewart as Dr. Otis Ford
- Jerry Wasserman as Dr. Yaeger Scanlan
- Samantha Ferris as Warden Stone
- Norm Sherry as Guard
- Dave Squatch Ward
- Wendy Chmelauskas as Laura Lang
- Ben Odberg as Lewis Lang
- James R. Baylis
- Michael Hanus
- Lindsay Bourne as Moira Sullivan
- "Falling" - Mindy Smith
- To scare means to fill one with fear.
- A scare also refers to a condition of alarm or worry.
- In the episode, a dangerous hallucinogen is released into the atmosphere of Smallville, causing a widespread "scare".
- The title could also refer to the "scare" effect the toxin has on its victims, causing them to live out their worst fears.
- Antagonist: The Kryptonite fear toxin.
- Clark uses the following abilities in this episode: Super-Strength, super speed, and heat vision.
- On the Season 4 DVD set, there is an alternate dream sequence for Lex in which he dreams that he is about to get married. It is then revealed that the person he is about to marry is actually his mother, who starts to blame Lex for his brother's death and warns him that he is on a path to destruction. Lex responds furiously by strangling his mother before being woken up by Clark.
- This episode is very similar to the chemical specialties of a villain from DC Comics called the Scarecrow. In Batman Begins, The Scarecrow and The League of Shadow used a "fear toxin" to poison the city's water supply, resulting in everyone in Gotham City facing their worst fears. Overall, the fear toxin used in this episode could allude to Batman's arch enemy Scarecrow, who uses a similar toxin that induces the same kind of fears into its victims.
- The same scenes of the nuclear strike can be viewed in Terminator 3: Rise of the Machines. The same scene is used in Apocalypse.
- When Chloe asks Clark what his worst nightmare was, instead of recounting what he sees in this episode, he describes the scenario shown to him by Cassandra Carver in Hourglass of losing all his loved ones and being alone, so he pretended that that was his "fear", caused by the Kryptonite fear toxin, instead of Lana discovering his secret and killing him, because he is guilty for her parents death.
- While under her hallucination, Chloe ran out of the room screaming and you can see four lockers: two red; two yellow. These look exactly like the lockers found at Smallville High. Unless Lex has similar lockers at his fertilizer plant, this scene was probably filmed inside the school.
- The fact that Clark wasn't one of the corpses in Lana's fear could be because she thinks he is invincible, or that he is no longer one of her loved ones. However, there are other bodies still covered, which may include Clark, Pete, Whitney Fordman, and Henry Small.
- Clark's fear depicts a second meteor shower. Whether it was planned to happen, or just Ironic is unknown but the season finale also shows a second meteor shower.
- Lionel tells the prison warden that he is guilty of many crimes, but not the one for which he was convicted. That is a lie on his part as he confessed to Chloe in Truth while under the effects of the Levitas project; a substance that makes people answer truthfully. This is odd behavior for a supposedly repentant man.
- Lilian Luthor's appearance in this episode was cut.
- Lex's "worst fear" contains elements of the vision of his future that Cassandra Carver saw in Hourglass.
- Dr. Scanlan also appeared in the episodes Resurrection, Vortex, Lexmas, and Nemesis. In Resurrection and Vortex, the doctor played by Jerry Wasserman was originally identified as Dr. MacIntyre, with Julian Christopher's character being called Dr. Scanlan.
- Wendy Chmelauskas and Ben Odberg reprised their roles as Lewis and Laura Lang for Lana's hallucination. They were last seen in the Pilot episode.
- Clark's worst fear is partially realized when a second meteor shower does indeed occur in Commencement.
- Lex's worst "fear" of an apocalypse is later realized in an alternate reality where a nuclear war destroys most of Earth in Apocalypse.
- As Chloe, Clark, and Lana are discussing Jason's fall, it is obvious that Chloe knows about the relationship between Lana and Jason. This is because after Jason's firing it was made public knowledge; which was stated by Lana when she confronted Clark in an earlier episode and confirmed in Pariah when Tim Westscott attempted to murder both Lana and Jason for what he deemed immoral behavior.
- With Clark's encouragement, Chloe visits her mother in the institution one year later in Tomb. The following year, it is revealed in Progeny that her mother's illness is a catatonic state induced by meteor infection.
- Lex's "worst fear" finally comes partially true when he becomes President of the United States in Finale, Part 2.
- Jonathan: Clark, I'm not sure that it's Lana, but I am sure that someday there'll be someone you can tell.
- Chloe: I can't get a statement from LuthorCorp. I mean, a huge chemical explosion causes everybody to relive their worst nightmares? What is that? And there's no sign of a lawsuit. I mean, it's like this whole thing... is just being completely forgotten about! What? What? What?
- Clark: I've seen you worked up before, but this is more than amped, even for you.
- Chloe: I guess it's just displaced anxiety. I found my mom, Clark.
- Chloe: I know that I gave you my word that I was going to stop all this prying, but... in this last few months I already got a taste of what it's like to keep my mom's secret, and I've never felt more alone.
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