NOTE: This spoiler was submitted by Jeremy

The film begins with Enola Holmes (Millie Bobby Brown) running from the local police. As she gets to an alleyway, she addresses the viewers and takes them back to how she got herself into this situation. Since her last adventure, Enola has started her own detective agency, but she has not been able to escape the shadow of her more famous brother Sherlock (Henry Cavill). Their mother, Eudoria (Helena Bonham Carter), is also still on the run, while Tewkesbury (Louis Partridge) is continuing his crusade for reform in England, and although Enola misses him, she says she has no time for distractions.

Enola is visited by a young girl named Bessie (Serrana Su-Ling Bliss), who asks her to locate her missing foster sister, Sarah Chapman. Bessie brings Enola to her home, next to a match factory where orphan girls are being forced to work. Enola meets Mae (Abbie Hern), an older, more abrasive girl that isn’t happy with Bessie going to her for help over Sarah. Enola looks for clues around the girls’ room and comes across a paper that says “12 March”, believing it to be a date. Bessie also informs Enola that Sarah had gotten into an argument with the factory’s foreman, Mr. Crouch (Lee Boardman), who says Sarah was stealing something from his office.

Enola follows Bessie to the match factory to pose as a new worker. She creates a diversion to sneak into Crouch’s office, where she sees the factory owners, Henry and Hilda Lyon (David Westhead and Roisin Monaghan), plus their son William (Gabriel Tierney), in a meeting along with Treasury Minister Lord Charles McIntyre (Tim McMullan) and his secretary Mira Troy (Sharon Duncan-Brewster). Enola finds a book with pages that Sarah tore out and took with her. She also finds old matches that had red tips instead of the white ones the girls work with.

At night, Enola follows Mae to The Paragon Theatre, where Mae works a second job as a stage performer with a troupe of other women. When Enola goes investigating in the dressing room, Mae confronts her with a knife, but she doesn’t give Enola any information she wants about what Sarah was doing in Crouch’s office. However, she does manage to bribe the stage manager into talking, as Sarah also worked there and was involved with a mysterious man. The manager gives Enola a poem that Sarah received. As Enola walks alone, she is followed by a man with a cane. She walks faster until she runs into Sherlock being thrown out of a pub, as he is too drunk to stand on his own and requires Enola’s help to get home.

After bringing Sherlock back to his home, Enola sees a board in his office over his latest case that has him stumped. In the morning, Sherlock deduces that Enola was working in the match factory due to the residue under her fingernails and he becomes concerned that she is in danger. Enola leaves and later runs into Tewkesbury in the park, where it’s clear they still have feelings for one another. She then reviews the poem again and finds that it’s a coded message that reveals a location in Whitechapel.

Enola arrives at the address and finds signs of a struggle. She then comes across Mae, dying from a stab wound. Despite her efforts to help, Mae doesn’t make it. On her person, Enola discovers sheet music titled “The Truth of the Gods”. She is then found by Inspector Lestrade (Adeel Aktar), who shows up with Superintendent Grail (David Thewlis), the man that was following Enola the night before. Grail accuses Enola of killing Mae, and when he continues to question her, she kicks him in the groin and makes a run for it, leading to the opening scene where she’s being chased by cops. She escapes and ends up hiding at Sherlock’s place again, just after Lestrade had paid him a visit. Enola questions him about his current case, and Sherlock admits it involves bribery and blackmail among government officials, but he cannot deduce the source of these crimes. When she brings him her findings, Sherlock decides to solve the Sarah case as well and urges Enola not to let her personal feelings get involved.

While both Holmes siblings look for clues, Sherlock goes to the apartment where Mae was murdered, and Enola finds clues based on the names of flowers (poppy was mentioned in the poem), and she finds the name “Sweet William”. Enola finds out a ball is taking place where she may find Sarah’s lover, so she goes undercover. When she tries to speak to the Lyons family, she is turned away for speaking without a chaperone. Enola ends up meeting Mira, who is kind to her and gives her a fan to impress the other guests. She runs into Tewkesbury and asks for his help in learning to dance. Upon further inspection, she finds that “Sweet William” refers to none other than William Lyon. She dances with him to try and get more information on Sarah, but William tells her people are watching them. At the same time, Tewkesbury dances with another girl Enola met named Cicely (Hannah Dodd). Meanwhile, Sherlock deduces that his clues have led him to decipher a message – “Good to meet you, Sherlock Holmes” – as well as finding a name from the account numbers he gathered – MORIARTY.

Enola is arrested at the ball in front of everyone and brought in for questioning by Grail. He tells her not to get involved any further in Sarah’s case, leading Enola to realize he is working for someone bigger. Sherlock goes to bail Enola out, but Grail attempts to hold her on his supposed evidence. Sherlock hits back with his own findings from the crime scene, figuring that one of the guards at the station might have been involved in Mae’s murder and seemingly figuring Grail’s own involvement.

Sherlock has no choice but to go to Eudoria and Edith (Susan Wokoma) for help, as Enola is being escorted to a women’s prison. The ladies stage a breakout and take Enola with them. On the ride out, the ladies use smoke bombs to get the pursuing guards off their tails. Enola talks to Eudoria about the case, having learned that the match girls at the factory were dying from what is allegedly typhus. Grail pursues and causes the ladies to crash their carriage. He attempts to have all three arrested, but they kick the guards’ butts and get away again. Enola then tearfully parts ways with Eudoria again so that she may complete her mission.

Enola returns to Bessie and finds nearby flower pots containing traces of red and white matchstick tips. She figures that the white tips contain phosphorus, which is the actual cause of death for the girls at the factory. Enola goes to Tewkesbury for help but has to hide when Cicely shows up, as she was already working with Tewkesbury on the case. Enola slowly realizes that Cicely is really Sarah and that William was her secret lover. Before they head out to continue to solve the case, Enola and Tewkesbury admit their love for one another.

Enola and Tewkesbury go to the factory, where Enola encounters Sherlock. He tells her that their two cases are connected, and he has found reason to prove her innocence in Mae’s murder. They find William dead in a chair, having been murdered as well. Enola finds clues suggesting Lord McIntyre had been conspiring with William’s father, saving money by replacing the match tips with the cheap phosphorus that’s been killing the girls. Using more clues from the sheet music, the siblings and Tewkesbury figure that they are being led to the Paragon Theatre.

The heroes meet Sarah at the theatre, where Enola has to break the news of William’s death to her. Sarah has a list of the girls who were killed by the phosphorus to make sure they receive justice. They are found by Grail and his goons, with Grail holding Bessie with a knife to her throat. Bessie bites his hand and runs free, leading to a fight between the heroes and villains. Sherlock and Tewkesbury fight the goons while Enola is chased by Grail. In the fight, Enola uses a hook to ensnare Grail by the groin and pull him upward, causing him to smash his head and break his neck against a wooden beam before he falls down dead.

McIntyre and his own goons show up with Lestrade, and he orders for Sarah to be arrested. Enola and Sherlock then gather all their information and conclude that the mastermind behind everything was Mira, AKA Moriarty (“Mira Troy” is an anagram). She attempted to blackmail McIntyre and the Lyons, but things got complicated when William stole their contract, and she hired Grail to help cover her tracks, which led to Mae and William getting killed for them getting too close to exposing them. Lestrade arrests Moriarty, but McIntyre burns the list of girls’ names to save his own skin.

Enola returns with Bessie and Sarah to the factory, where they expose the girls’ deaths and lead a revolt against Crouch and the other employers. The girls all walk out in protest, which Eudoria and Edith watch and know that Enola is responsible.

Sherlock visits Enola and presents her with a newspaper saying that Tewkesbury has helped gather enough evidence to have McIntyre arrested. While Sherlock offers her a partnership, Enola declines but agrees they should meet every Thursday at 4. As she leaves, Sherlock is met by Edith, who gives HIM a newspaper that shows that Moriarty has escaped custody and is on the loose. Enola then goes out on a date with Tewkesbury.

During the credits, a man (Himesh Patel) shows up to Sherlock’s home to answer an ad for a flatmate (Enola’s idea). Despite some hesitance, Sherlock invites the man in. When asked his name, the man replies “Doctor. Doctor John Watson.”

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Enola Holmes is trying to establish herself as a professional detective but cannot seem to escape the shadow of her more famous brother Sherlock. She is found by a young girl named Bessie, who asks Enola for help in finding her foster sister Sarah Chapman. With help from Sherlock and Tewkesbury, Enola is on the case.

Enola learns that Sarah had uncovered a horrifying truth that girls at the match factory where she and Bessie work were dying from what was allegedly typhus, but is actually phosphorus from cheap match tip materials that are poisonous. Enola initially suspects William Lyon, son of the factory's owners, but he, along with another foster sister, Mae, are found murdered. Enola is pursued by Superintendent Grail, a man of the law with dishonest intentions. He arrests Enola on the charge of Mae's murder, but she is broken out when Sherlock seeks help from their mother Eudoria and her companion Edith.

Enola finds out that a young woman named Cicely that was working with Tewkesbury is Sarah, and that William was her lover trying to help expose the match factory. The heroes are led to the Paragon Theatre, where Sherlock shows up since the blackmail case he has been working on is connected to Enola's case. Grail and his men confront the heroes, and Enola causes Grail to be killed when he gets caught in a hook and is pulled up to have his neck broken by a wooden beam. Lord McIntyre, who worked with William's parents, shows up, and it turns out that his secretary, Mira Troy, AKA Moriarty, is the true mastermind, having hoped to blackmail McIntyre and the Lyonses, only for William to get involved, so she hired Grail and had William and Mae killed. Moriarty is arrested, but McIntyre seemingly escapes justice by burning the evidence of the girls' names who were killed by the factory's conditions.

With help from Tewkesbury, McIntyre is arrested, but Moriarty escapes custody. Enola continues to try and make a name for herself separate from Sherlock, and she begins a relationship with Tewkesbury. Meanwhile, Sherlock meets a young man hoping to be his flatmate - Dr. John Watson.