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Smallville stands as the longest-running TV series based on a DC (or Marvel) property ever made, running for a staggering 217 episodes across 10 seasons of television that aired from 2001 to 2011 on The CW. That's a colossal achievement, offering fans the fullest depiction of the DC universe they've yet to see in live-action, despite the best efforts of Zack Snyder's DCEU. Even so, a TV show can't run that long without repeating itself a few times, and one of Smallville's most notorious tropes is its reliance on characters developing convenient amnesia.
Clark Kent protecting the truth about both his alien origins and his array of superpowers provides the dramatic backbone of the series, serving as the primary obstacle in his romances with Lana Lang and Lois Lane, as well as his friendships with Chloe Sullivan and, most notably, Lex Luthor. Lex's obsession with discovering what his super-secretive friend is not telling him is one of the main reasons he ends up as Clark's ultimate foe. However, there were many times he almost found out the truth, until some form of unlikely twist or turn restored the status quo and erased Lex's memories of his discovery. This trope may have frustrated fans, but there was a very good reason for it.
Lex Luthor Learned Clark's Secret Over and Over, But Forgot The Truth Every Time
Smallville Used Numerous Plot Devices to Keep Lex in the Dark
Clark and Lex Luthor first met in Smallville's very first episode, "Pilot," when Lex accidentally hits Clark with his car, only for Clark to save his life when he subsequently plows into a river. His investigation into how Clark could've survived becomes a burning passion, but it wasn't until two years later that Lex (briefly) found out the truth.
In Season 3, Episode 8, "Shattered," Lex witnesses Clark using his superhero powers with his own eyes when Clark steps in front of a speeding car to protect him. Unfortunately for Lex, this incident coincides with an elaborate scheme concocted by his father, Lionel Luthor, to manipulate his son's memories, after Lex discovered evidence that Lionel once murdered his own parents for insurance money. The episode ends with Lionel committing Lex to Belle Reve Sanatorium. In the following episode, "Asylum," Lex is subjected to electroshock therapy, which erases his mind of both his father's crimes and Clark's secret.
In Season 5, Episode 12, "Reckoning," Clark finally tells Lana everything, and the pair get engaged. Deducing that she must now know Clark's secret, Lex demands she tell him the truth and won't leave her alone until she does. In their subsequent car chase, Lana is killed in a crash, and Lex witnesses Clark super-speeding to her side. However, his discovery is short-lived as Clark implores Jor-El to use the powers of the Fortress of Solitude to reverse time so he can undo Lana's death by continuing to withhold his secret.
"Reckoning" was the landmark 100th episode of Smallville and also featured the death of Jonathan Kent.
The next time Lex realizes the truth also involves the Fortress. In Season 7's finale, "Arctic," Lex has at last figured out Clark's alien nature after getting his hands on a Kryptonian Orb and discovering the location of the Fortress. Lex uses the Orb to bring down the Fortress, collapsing it on top of them. This proved to be star Michael Rosenbaum's final regular appearance in Smallville, but in Season 8, it is confirmed Lex survived, albeit badly injured. Oliver Queen ultimately kills him, with the show using a stand-in actor, in Season 8's "Requiem."
Smallville Season 10 later revealed that Lex had started a secret cloning project prior to his demise, in a bid to resurrect himself in the event of his death. In the series finale, Lex was finally revived and took back control of Luthorcorp, murdering his sister, Tess Mercer, in the process. She, however, got her revenge when she injected him with a neurotoxin in her dying moments, which erased his memories one more time for the road.
Lex Knowing Clark's Secret Would've Changed More Than Just Smallville
The Writers Wanted to Maintain DC's Iconic Superman Mythology
In terms of the behind-the-scenes logic for why Lex was never allowed to remember the truth about Clark in Smallville, the answer is likely that the writers simply did not want to shake up the show's winning formula. However, there is another deeper reason for the repeated bait-and-switch plotline. If Lex had ever become fully aware of Clark's secret identity, it would constitute an alteration to the traditional story of Superman that would have had huge ramifications for the Man of Steel's crime-fighting career, expanding far beyond the confines of Smallville's exploration of Clark's early years.
If Lex had retained his memories of learning Clark's secret during Season 3, then this no doubt would've strengthened their friendship and given him a new purpose as Clark's confidante and staunch ally. In the long run, this would possibly have meant that he never continued down his darker path and his destiny as Superman's nemesis would've been erased. Conversely, if Lex — by then a fully-fledged villain — had survived his confrontation with Clark in the Fortress in Season 7, he may well have defeated his enemy before he even got the chance to don his cape and become the protector of Metropolis.
Lex's memory-wipe in Smallville's series finale irks fans the most, as some feel that such a move was unnecessary because the show was ending anyway. However, the finale concluded with Clark finally making his public debut as Superman, and so keeping Lex unaware was more important than ever. If Lex knew all of his former friends' secrets, he would be well-equipped to hurt everyone Clark cared about and destroy both of his identities, ending the Man of Tomorrow on day one and erasing the iconic DC mythology. Lex may have wanted nothing more than to find out the truth about Clark, but for the sake of Superman's legacy, he was doomed to always be left in the dark.
