The new Prime Video series Young Sherlock takes a big risk with Sherlock Holmes lore. Instead of giving the teen detective his usual partner Dr. John Watson, the show pairs him with his future arch-nemesis, James Moriarty. The change is surprising, but it works. The series, which premiered March 4, 2026, makes a strong case that Moriarty is actually the better sidekick for a young, inexperienced Sherlock.
All eight episodes are streaming now on Prime Video, and viewers are seeing a completely different dynamic between the two famous characters. Here is why this bold choice pays off.
A Friendship Built on Equal Genius
The biggest reason Moriarty works as a sidekick is simple: he is Sherlock’s intellectual equal. In the original stories by Sir Arthur Conan Doyle, Professor James Moriarty is a mathematical genius and criminal mastermind who matches Holmes step for step . Young Sherlock uses that equal footing from the very beginning.
In the series, Sherlock Holmes (played by Hero Fiennes Tiffin) meets James Moriarty (Dónal Finn) at Oxford University. Sherlock is working as a scout, a housekeeping staff member, while Moriarty is a scholarship student . When both are blamed for a crime they did not commit, they team up to clear their names .
Their partnership works because they challenge each other. Moriarty pushes Sherlock to think differently. The showrunner Matthew Parkhill explained the relationship to ScreenRant, calling it “so beautiful.” He said, “I don’t want them to rupture too soon, because it’s just going to break my heart. So I’m going to want to enjoy that friendship” . The friendship feels real because both are brilliant, but in different ways.
A review from Variety noted that their dynamic is “the most engaging” part of the series. It explained that as their bond strengthens, their different moral codes start to show. James is loyal to Sherlock, but he “will always look out for his own interests first” . That tension makes every scene between them exciting.
Moriarty Fills the Gaps Watson Cannot
In the traditional stories, Dr. John Watson is a steady, loyal, and brave former soldier. He is the heart that balances Sherlock’s cold logic. But in Young Sherlock, the 19-year-old detective needs something different. He does not need a calming influence yet. He needs a partner who can keep up with his chaotic energy.
Guy Ritchie, who directs and executive produces the series, is known for focusing on action and banter between partners . In this version, Moriarty provides that banter. The Hollywood Reporter review pointed out that Watson is “badly needed” in the story because Moriarty is played as “Sherlock Holmes, Only Louder.” The reviewer felt the contrast Watson offers is missing .
However, other critics disagree. Times Now praised the duo, stating that Hero Fiennes Tiffin and Dónal Finn “make for a good duo.” The review highlighted that Finn “does a great job of adding ambiguity into his friendship with Sherlock, with strains of what could lead to him later becoming his greatest nemesis” . That ambiguity is something Watson could never provide. Watson is purely good. Moriarty is a question mark.
Watson is also traditionally a man of action who can handle himself in a fight. In Young Sherlock, that role goes to the Chinese princess Shou’an (Zine Tseng), who handles most of the martial arts sequences . This frees up Moriarty to be the witty, intellectual sparring partner.
The Tension of a Doomed Relationship
Audiences watching Young Sherlock already know where this friendship ends. They know that Moriarty will eventually become the “Napoleon of Crime” and Sherlock’s greatest enemy . That knowledge hangs over every scene they share.
This creates dramatic tension that a Watson partnership simply does not have. When Sherlock and Moriarty joke around or solve a case together, viewers are waiting for the other shoe to drop. The show plants these seeds carefully. Parkhill confirmed to ScreenRant that “the seeds have been planted” for Moriarty’s turn to the dark side. He added that “ultimately, yes, the decline of that friendship will be a key part of the story” .
The series borrows from the Yuukoku no Moriarty anime concept, where the two geniuses work together as partners. In that anime, they function as “two sides of one brilliant mind, playing off of each other” . Young Sherlock takes that idea and runs with it, showing that if they were not destined to be enemies, they would be unstoppable friends.
The series also pulls from the Andrew Lane Young Sherlock Holmes novels, which reimagine the character’s early years . In those books, Sherlock faces adventures that shape who he becomes. By making Moriarty his first real friend, the show guarantees that his eventual betrayal will hurt more.
A Fresh Take on a 138-Year-Old Story
People have been adapting Sherlock Holmes since 1887 . Viewers have seen dozens of versions of Watson, from Jude Law in the Ritchie films to Martin Freeman in the BBC series Sherlock . By removing Watson entirely, Young Sherlock forces itself to be different.
The show is not a prequel to the Robert Downey Jr. films. Parkhill made this clear, stating, “It’s not a prequel… He doesn’t grow up to be Robert Downey Jr.” . It exists in its own world. In this world, a teenage Sherlock does not need a war veteran documenting his cases. He needs a friend his own age who understands what it feels like to be an outsider.
Dónal Finn plays Moriarty with full energy. He is charismatic, smart, and slightly dangerous. Even when he is helping Sherlock, there is something unpredictable about him. The Hollywood Reporter criticized the character as “unformed and unlikable,” but admitted the writing was the issue, not Finn’s performance . Other outlets praised the choice, with NDTV noting that the story begins when Sherlock meets Moriarty, “setting up a rivalry that will shape his future” .
The series also includes familiar names like Lestrade and nods to future details, such as a joke about the deerstalker hat . But at its core, it is about the friendship between two young men who will one day try to kill each other.
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What This Means for Future Seasons
The first season of Young Sherlock ends with the duo’s friendship intact, though cracks are starting to show. If the show returns for a second season, the producers will have to decide when to introduce Watson.
Parkhill teased the idea carefully. He mentioned that introducing Watson would create a “love triangle” between Sherlock, Moriarty, and the new partner . This suggests Watson’s arrival could be the breaking point that pushes Moriarty away.
For now, the show proves that Moriarty is more than just a villain. He is a compelling partner who makes Sherlock better while secretly planting the seeds of his own destruction. The series gives fans something they have never seen before: the world’s greatest detective and his greatest enemy as best friends.
Young Sherlock is streaming now on Prime Video.
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