Dexter: Original Sin season 1 episode 9 ending explained: Who left a note on Captain Spencer’s door?

Dexter: Original Sin season 1 episode 9, titled Red Mist, aired on February 7, 2025, continuing the escalating battle between Dexter Morgan and his growing list of enemies. As the prequel series explored Dexter’s early years, it delved deeper into his struggle with his dark urges while Harry tried to guide him.

Meanwhile, Brian Moser’s presence became increasingly menacing, pushing the two brothers closer to a dangerous confrontation. The episode delivered shocking twists as Dexter turned his focus to Captain Spencer, suspecting him of involvement in something sinister.

At the same time, Brian continued his violent revenge against those who separated him from Dexter. As the story unfolded, both Dexter and Harry uncovered disturbing truths, leading to an intense showdown.

In the final moments, a mysterious note appeared on Spencer’s door, raising new questions about who was really in control. However, it was Dexter himself who left the note, using it as part of his plan to make Spencer reveal Nicky’s location. Misinterpreting the situation, Spencer assumed that Victor, whose identity remained unknown, was the real culprit. Ultimately, Dexter’s plan worked exactly as intended.

Maria and Harry (Image via Paramount Plus)

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Dexter: Original Sin season 1 episode 9: Dex laid Spencer on his victims' table

Dexter carefully planned his trap for Aaron Spencer, ensuring the police captain would fall right into it. Convinced that Spencer was responsible for Jimmy Powell’s murder and Nicky’s kidnapping, he crafted the perfect setup to catch him.

Dexter allowed Spencer to think he had escaped, all while tracking him to find Nicky’s location. Their big showdown happened in an arcade, where Dexter managed to take Spencer down, even though he was weak from donating blood earlier.

Dexter Morgan (Image via Paramount Plus)

After knocking Spencer out with M99, Dexter tied him to a table, putting him in the same position as his victims. He then pressured Spencer to tell the truth about Nicky. But Spencer refused, warning that if he died, Nicky would be left to starve. Furious, Dexter cut off Spencer’s finger, which was unlike his usual careful approach.

However, he made sure to loosen Spencer’s bindings on purpose, letting him think he had escaped. Now, as Spencer unknowingly leads Dexter to Nicky, the stage is set for a final showdown in the season finale.

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Dexter: Original Sin season 1 episode 9: Harry gets a warning about Brian Moser

Harry Morgan had long suspected that the NHI Killer was someone from his past, but in Dexter: Original Sin episode 9, he finally learned the shocking truth that Brian Moser had been stalking Dexter.

While looking through old crime scene photos, Harry noticed something chilling, Brian was in the background at multiple murder scenes, watching Dexter work. This proved that Brian’s murders were personal and that he was targeting the people who had kept him away from his brother.

Brian Moser (Image via Paramount Plus)

As Harry struggled to process this truth, his old friend and partner, Bobby Watt, suddenly woke up from his coma. Even though he was weak, Bobby managed to say something urgently, "Save Dexter."

This warning was important because it showed that Bobby, even in his fragile state, understood how dangerous Brian was. It was unclear what Harry would do next, but since Brian had been killing those who got in his way, Harry could be his next target. With Brian getting more reckless and Dexter still unaware that his brother exists, the tension was at its highest.

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Other highlights of Dexter: Original Sin season 1 episode 9

Episode 9 of Dexter: Original Sin had several important moments that gave more depth to the characters’ backstories. One of the biggest reveals came in a flashback where Harry convinced Captain Aaron Spencer to allow a search for Laura Moser.

To make this happen, Harry had to admit that he had an affair with Laura, which cost him his detective badge. However, this led to the unforgettable moment when Dexter and Brian were found in the blood-soaked shipping container, changing their lives forever.

Dexter and Tanya Martin (Image via Paramount Plus)

Another key moment focused on Debra Morgan. After donating blood at Miami Metro, she met Tanya Martin, a dedicated police officer. Tanya told Deb that catching criminals gave her the same excitement as playing sports, which influenced Deb’s decision to become a cop.

Meanwhile, a flashback shows how Harry and Doris briefly tried to raise both Dexter and Brian, but things went wrong when Brian tried to kill baby Deb. This terrifying incident forced the Morgans to give Brian up, which later led to his desire for revenge.

As the episode continued, Brian’s presence became more threatening, and Harry realized that his long-lost son had been secretly watching Dexter for years.

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Fans of the show can watch Dexter: Original Sin season 1 episode 9 on Paramount+ Showtime.