- For the kryptonian, see Black kryptonite.
After a kryptonite explosion, Lex is "split into two" with Lex as his good side and Alexander as his evil side. Alexander immediately imprisons Lex in the mansion and begins his rampage by trying to kill Clark and Chloe in the lab, fencing with Lionel and convincing him to return to his dark seeded nature, and finally, threatening to close the Talon unless Lana moves in with him.
→ see also Category:Screencaps from episode 4x17
Lex and Dr. Sinclair are inside a LuthorCorp lab about to undergo an experiment involving green kryptonite. In the experiment, the kryptonite must be super-heated. Unaware of the risk, Lex and Sinclair go ahead, but begin to worry when the kryptonite exceeds its target temperature. After an alarm sounds, Lex shoves Dr. Sinclair out of the way while he is caught in a burst of energy emitted from the kryptonite, which has transformed to black kryptonite. This produces an effect where Lex is split in two - one of which is his darker alter ego. Shortly after falling to the floor, Lex - unaware of his split self - helps up the scientist and brings him to the hospital. Watching the two make off is the evil Lex who is just realizing what has happened.
Clark and Chloe go to visit Lex (Alexander) in the hospital and begin to ask what happened. Alexander says that he'll take Clark to the lab and show him what he was doing. Clark accepts, but Chloe says that she'll stay behind to ask the scientist a few questions.
Before Dr. Sinclair is visited by Chloe, Lex visits him and tells him what happened and asks if there's a way to reverse the process. After the scientist asks for his notes back from the lab, Lex puts air into his IV tube, thus killing the scientist. Lex then exits the room to find Chloe who wonders why Lex is there rather than at the lab with Clark. She asks if she can see the scientist. However, Lex says that he's sleeping and she has to come back later. Lex then walks away when a code blue is alarmed for the scientist's room and Chloe realizes that the scientist is dead.
At the LuthorCorp lab, the good Lex shows Clark around and explains that he has been conducting experiments with plants, trying to get them to grow in the most harsh environments. He gives Clark an apple, but it tastes rotten. Lex explains that even if the plants grow anywhere, they don't retain their usual taste. Then Lex opens a locker containing kryptonite samples and Clark excuses himself, blaming the apple for his nausea attack. In the background, Alexander has overheard the whole conversation.
Later at the Luthor Mansion, Lex is visited by his father, who is arranging a charity fund raiser. Lex agrees to come to the fund raiser, on the condition it's held in the LuthorCorp Plaza. Lionel is surprised by Lex's newfound generosity. When Lionel leaves, Alexander walks in and knocks Lex unconscious. Then he brings him to the cellar, and imprisons him there. He explains he does not know what might happen if he killed him.
At the Torch, Chloe informs Clark that the scientist died at the hospital. They realize that Lex was at the lab and in the hospital at the same time. Chloe then retrieves a surveillance movie from the hospital security system, showing Lex being in two places at once.
Lex goes to Lana's apartment. He tells her she deserves better than Jason. He then kisses her and tells her she can be his queen. Lana slaps him and orders him out. Lex then tells her that he's going to close the Talon.
At the mansion, Lex is practicing fencing. Lionel comes into the room. He asks him how he could invite him to the LuthorCorp Plaza and then cancel the thing behind his back. Lex then quotes Lionel back to him: "Because helping those who refuse to help themselves is a waste of time, and money, son!" He challenges Lionel to a round of fencing, and attacks him with his rapier while taunting him, trying to get the old Lionel back. After a while, Lionel reverts to his old self and overpowers him. Lex walks out of the room, happy to see his father revert to darkness.
Clark walks in on evil Lex, who tricks him into thinking he is the good Lex by talking about their trip to the lab. Clark explains about the split. Lex acts surprised and tricks Clark and Chloe into the lab. Once there, he drops a large block of concrete on top of them. Clark protects Chloe from the falling debris and brings her to the house, while Lex watches. At the house, Clark tries to get Chloe to the hospital but she convince him that she is fine. Chloe questions how they were able to survive, but Clark refuses to come clean.
In the barn, Clark encounters Lex, who tries to coerce him into joining him so they can rule the world together. Clark declines and grips his throat. Lex puts on a kryptonite ring and tells Clark that he knows his weakness. Martha and Jonathan come in and Lex shoots Jonathan in the leg and departs.
After taking Jonathan to the hospital, Clark goes to the mansion searching for good Lex. Using his super hearing, he hears his moans and super speeds to free him from the wine cellar. However, evil Lex expects that and overpowers Clark using his kryptonite ring. Lex and Alexander fight until Clark uses his heat vision to turn the evil Lex's kryptonite ring into black kryptonite, fusing the two Lexes together.
After the incident, Lex doesn't remember anything from the Alexander's memories - including Clark's secret. Lex apologizes to Clark and Lana. When he comes back to the mansion, Lex finds Lionel to be his old self. Lionel says he has seen the error of his ways and that he'll close the charity foundation immediately. Lex protests, but Lionel replies, "A man can't deny his true nature. We're Luthors... we're Luthors". With the cryptic and dark smile on Lex's face after Lionel's statement, it's revealed that Lex's dark side isn't as hidden as Clark thinks.
- Tom Welling as Clark Kent
- Kristin Kreuk as Lana Lang
- Michael Rosenbaum as Lex Luthor
- Jensen Ackles as Jason Teague (credit only)
- Allison Mack as Chloe Sullivan
- John Glover as Lionel Luthor
- Annette O'Toole as Martha Kent
- John Schneider as Jonathan Kent
- "I Drive Alone" - Esthero
- Onyx is a mineral gemstone with a long history of use for hardstone carving.
- The title refers to black kryptonite, a newly made mineral that originates from green kryptonite.
- Antagonist: Alexander Luthor
- Clark uses the following abilities in this episode: heat vision, super speed, super strength and super hearing.
- Jason Teague does not appear in this episode. This is the first of two episodes of Season 4 in which Jason does not appear. The other is Ageless.
- The production crew and fan sites refer to the "evil" Lex as Lex, and the "good" Lex as Alexander, presumably referring to the fact that Lex's loving, warm, and good-natured mother referred to him as Alexander while his derogatory father always refers to him as Lex. Both incarnations are referred to as Lex on screen.
- Shelby appears in a deleted scene of this episode.
- If counting "Dream Lex" in Slumber, this is the fourth time that Lex has discovered Clark's secret. The other times were when he saw Clark use his abilities in Hug, but forgot after the effects of Bob Rickman's handshake wore off, and in Shattered when Clark stopped Morgan Edge's car from running him over only to have his memory wiped by electro-shock therapy in Asylum.
- In this episode, when Clark used heat vision on green kryptonite, it turned into black kryptonite. However, when Clark used heat vision on green kryptonite in Whisper, the beam of heat was reflected back at his eyes and blinded him. The blinding effect may have been due to the particular cut of that episode's piece of kryptonite or the particular way in which Clark was using his heat vision then (in intense bullet-like bursts rather than the slow, steady beams used in this episode).
- "Evil Lex" challenges Lionel to a fencing duel in order to see whether Lionel is still worthy to be his father. This is reminiscent of the duel in which Lionel challenged Lex in on Hothead.
- "Evil Lex" says to Clark that if they were to team together they could rule the earth like "Gods among men". This is another direct reference to the message in Clark's ship that was revealed in the episode Rosetta.
- This episode marks Lex and Lana's first kiss.
- This is the second episode to feature black kryptonite, the first being Crusade.
- This episode marks the 21st appearance of Clark's red jacket & blue shirt outfit, which he wears frequently throughout the series.
- "Evil Lex" said: "Oh, by the way I'm closing the Talon". He wouldn't be able to do this since Lex made sure it was impossible for him to do so in a previous season. (Though Lana did sell her ownership of the Talon to Lex to go to Paris, he could be the only owner now.)
- Evil Lex's offer for Clark to join him and rule the world makes another parallel between them and Devilicus and Warrior Angel.
In Other Media
- When Evil Lex gives his monologue about joining him and ruling the world, he says, "I want you to remember this day, Clark. I want you to remember that despite all your amazing powers, there was one man that could beat you." This parallels a famous monologue of Bruce Wayne/Batman in the famous Dark Knight Returns, where he says, "I want you to remember Clark, in all the years to come, in your most private moments, I want you to remember my hand at your throat. I want you to remember... The one man who beat you."
- Lex is seen wearing a kryptonite ring. In the comics, Lex wears a kryptonite ring and as a result, he loses his hand due to kryptonite poisoning.
- The fight between Clark and Lex (Alexander) mirrors both the comic counterpart's fights and both Clark and Lex's future.
- The WB promoted this episode as "Lex vs. Lex".
- Working titles for this episode were "Lexes" and "Fissure." 
- Michael Rosenbaum tells the story at panels that he had told Tom Welling he should have been paid twice for this episode since he played "two characters." Tom laughs back that that is not how it works.
- Clark will save Chloe in a very similar manner as in this episode in Blank.
- Another kryptonite ring created by Lex resurfaces four years later in Season Eight. Once in Requiem, when Oliver steals it from Lex, and once in Injustice, when Oliver uses it to stop Parasite (who stole Clark's powers earlier in the episode). However, this ring has a slightly different design, as it is now in the form of a signet ring with the LuthorCorp insignia engraved on it.
- The idea of a "good Alexander" is portrayed in Fracture, and also in Season Ten.
- It could be argued that when the two halves of Lex were merged at the end, that his evil side remained dominant and thus this is the episode where Lex began to truly change from good to evil. Lex's smirk in the end when Lionel reveals he's cancelled the charity he was going to start could indicate this. In his remaining Season Four episodes, Lex is noticeably more unscrupulous than in the past, particularly in Ageless and Commencement. His friendship with Clark will come to a permanent end early in Season 5, and soon after he will deliberately torture Arthur Curry in Aqua, which would seem to go beyond the pale of any good justification he could come up with.
- When evil Lex forces an iron-like mask on good Lex, it's a reference to the story "The Man in The Iron Mask" where the main character has an identical twin who is kept in prison while the other is free. Furthermore Evil Lex says "it's good to be the king" also relates to the story where one of brothers becomes king and the other is in prison.
- When evil Lex says "Hello me, meet the real you" it is an allusion to the Megadeth song "Sweating Bullets" in which a schizophrenic talks to himself saying, "Hello me, meet the real me."
- Chloe: Well, I was hoping I could maybe talk to him.
- Alexander: Well, he's sleeping now. Maybe you should come back later when he's feeling a little more alive.
- Alexander: Just like in that Dumas story. It was always one of our favorites, wasn't it? King Louis XIV imprisons his twin brother in the Bastille forcing him to live out his final days behind an iron mask. See, most people think Louis did it to hide his brother's true identity, but we know better. He did it because he couldn't stand looking at such a pale reflection of his own greatness. (Alexander places the mask over Lex's head) It's good to be the king.
- Clark: Well, there must be way to figure out the time-line.
- Chloe: Yeah, there is. Um, Smallville Medical Center has a surveillance system which, as far as you know, I've never hacked into, because... that would be... wrong.
- Clark: Where's Lex?
- Alexander: He's standing right in front of you.
- Clark: No, you're not him. The real Lex would never try to kill me or Chloe.
- Alexander: No, you're probably right. But he's thought about it! All the times you've meddled in his plans, derailed his ambitions. He's thought about killing all of you - he just never had the guts to go through with it.
- Martha: What do you want?
- Alexander: The world, Mrs. Kent.
- Alexander: You were right about me all along, Mr. Kent. I am the villain of the story.
- Lex: Leave him alone.
- Alexander: I'm starting to hate the sound of my own voice.
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