As the film opens, Enola Holmes rides her bicycle and narrates to the audience. She explains that her mother Eudoria, keen on word games, insisted on naming her Enola (a reverse of the word Alone) after her birth, and that following the early death of her father and subsequent departure of her elder brothers, Mycroft and Sherlock, her mother raised and educated her in their country home of Ferndell Hall. Eudoria raised Enola on many books, sciences, and sports, and encouraged a free and playful lifestyle, unlike the stuffy and traditional schools and expectations of Victorian England. Despite this usual closeness, Enola once accidentally interrupted a secret meeting between her mother and a group of other women, who hid their intentions from her. She goes on to explain that on the morning of her sixteenth birthday, Enola awoke to find her mother missing, leaving only a wooden gift box with a booklet on flowers, cards, and pencils as a birthday present passed on to her by their housekeeper, Mrs. Lane. Back in the present, Enola tells the audience that a week has passed since Eudoria's disappearance, and she is on the way to the train station to receive Mycroft, now a rich aristocrat working for the government, and Sherlock, now the famous detective. Upon their arrival, her brothers fail to recognize her at first, having not seen her since she was small. Mycroft is horrified that Enola is out in public without a hat or gloves, and dirty from having fallen off her bike on the way to the station. He is further annoyed to find that Enola has not brought the carriage he has been paying for, confusing Enola as they do not own a carriage. Sherlock hires one to take them to Ferndell, and upon their arrival, Mycroft balks at the state of the house. He tells Sherlock that he has been sending a great deal of money to Eudoria, who sent letters to him detailing the costs of the household, including rooms, members of household staff, and teachers for Enola, which do not exist. Sherlock investigates, and surmises that although Eudoria's absence is not foul play, she does not intend to return. Mycroft and Sherlock discuss what to do with their younger sister now that Eudoria is gone. Mycroft feels that Eudoria has robbed him, and saddled him with Enola. Sherlock thinks Enola should be allowed to stay at Ferndell, as Eudoria educated her on virtually every subject herself, and she seems to be very clever. However, Mycroft insists on sending her away to a strict boarding school so she may be turned into a proper lady, citing that she is his ward now, and Sherlock has shown little interest in the family. Mycroft sends for Miss Harrison of Miss Harrison's Finishing School for Young Ladies, and Enola's free spirit and defiant attitude gains her a slap from the headmistress. Enola begs not to be sent away, but Mycroft is adamant, and Sherlock refuses to help. That evening, Sherlock finds Enola sketching and recounts the only memories he has of her, including a pine cone wrapped in wool she called Dash, after one of Queen Victoria's dogs. Enola chastises Sherlock for never visiting or writing home, telling him she has followed every one of his cases. Sherlock sees Eudoria's vanishing as an interesting mystery to be solved, and tells Enola to look for what's already in front of her. That night, Enola finds a secret coded message from her mother in the gift box she left. She decodes it and follows the clues in her booklet to a hidden envelope filled with money addressed to her, with the only note saying: "Our future is up to us." Enola decides to run away to search for her mother, dressing as a boy and boarding a train bound for London. A rich family is at the station, frantically looking for someone. Enola notices a latecomer on the train wearing a brown bowler hat who gives a signal to someone on the platform. The train leaves and is well on it's way when a boy Enola's age pops out of a large travel bag in her compartment. Enola deduces that this is the boy the family at the station was looking for, and he introduces himself has the Viscount Tewkesbury, Marquis of Basilweather, and says that he is running away. Enola tells him of the man in the bowler hat who she has also deduced has been sent to look for him. Sure enough, the Bowler Hat man is approaching their carriage, and Enola leaves, not wanting to get involved, as the man passes her and finds Tewkesbury. She hears the boy scream, and returns to find the man holding him outside the train door, about to smash him into a tunnel wall. Enola attacks the man, Linthorn, and saves Tewkesbury, jumping off the train just before a bridge to strand the attempted murderer.Sherlock and Mycroft discover Enola has run off, angering Mycroft. Mrs. Lane tells Sherlock that Enola knows nothing of the outside world in practicality, and asks him not to abandon her a second time. Enola and Tewkesbury begin to walk across the countryside toward London, stopping to rest at dark, where Tewkesbury reveals his vast knowledge of plants, flowers, and edible fungi to forage for food. Enola cuts his long hair short to help him not be recognized, as he explains that, after narrowly avoiding being crushed by a falling tree branch, he realized that he didn't want to join the army like his uncle as his family wants him to, or take his seat in the House of Lords like his deceased father. Enola tells him in turn of her decision to run away rather than be sent to Miss Harrison's. Tewkesbury proposes they stick together in London, but Enola refuses, and they go their separate ways upon reaching the city. Using the money Eudoria left for her, Enola disguises herself as a well-bred lady, and finds a boarding house to stay at. Meanwhile, Sherlock and Mycroft return to London. Mycroft has the police on the lookout for Enola, and Sherlock has turned down the Tewkesbury case in favor of searching for her and Eudoria himself. Mycroft is insistent that their name be kept out of it, not wanting to be embarrassed by Enola, and complains about an upcoming vote to reform voter rights by the House of Lords, which is at a deadlock. Enola leaves a coded message in various newspapers for her mother, finding that the Viscount Tewkesbury's disappearance has gotten a lot of press. She then tracks down the correspondent her mother often wrote to, Ethel, who runs Jiu-Jitsu classes for women above her tea shop and used to teach Enola self defense. She tells Enola that Eudoria does not want to be found and has secret work to do in London, and Enola remembers her as one of the women at Eudoria's secret meeting. Enola finds fireworks in Ethel's office after she returns to her students, and recalls hearing coded words in their conversation at the meeting. Decoding them as locations, she surmises that they are hiding something at the docks, and goes to investigate. Once there, Enola finds a warehouse bearing a purple ribbon, the same ribbon as was on her birthday gift from her mother. Sneaking in, she finds great quantities of fireworks, gunpowder, and other explosives. She also finds posters for suffragette rallies, and is horrified to find full fledged bombs hidden away. She questions weather she should continue her attempts to find her mother, as it seems she is involved in dangerous activities. As she goes to leave the docks, Enola is ambushed by Linthorn, who interrogates her about Tewkesbury's location and attempts to drown her in a water trough. She outsmarts him, momentarily getting away, and explains to the audience how he mother taught her various subjects, including physical combat. An intense fight between her and Linthorn ensues, with Enola eventually being overpowered by Linthorn's superior strength. He throws her through the warehouse door and attempts to finish the job and stab her, though she is protected by her whalebone corset. She sets off the gunpowder in the room and escapes Linthorn. While nursing her wounds, Enola tells the audience of how she once almost died trying to save a sheep, and her mother's advice not to risk her life to help others. However, Enola decides she cannot leave Tewkesbury to his fate, and resolves to find and help him.Enola once again disguises herself, this time as a widow named May Beatrice Poesy, in order to gain a meeting with Lady Tewkesbury, the marquis' mother, at Basilweather Hall. Tewkesbury's Mother, Uncle, and Grandmother are all skeptical of Enola's claim of being a lady detective, until she says she works for Sherlock Holmes as an advance investigator. This piques their interest, though Inspector Lestrade of Scotland Yard enters and says that Sherlock always works alone. Tewkesbury's Uncle, Sir Whimbrel, sends them both away, stating that they do not need more help. Enola and Lestrade test each other on their knowledge of Sherlock, finding themselves equal. Lestrade leaves, confused by 'May'. Enola pays a gardener for his uniform, and is pointed towards the woods as the place where Tewkesbury spent most of his time. Lestrade meets with Mycroft and tells him of 'May Beatrice Poesy' who matches the description of Enola. Mycroft confirms she is his sister, and tells Lestrade to find her. In the forest, Enola finds the tree branch that almost killed Tewkesbury, and discovers saw marks on it's base. She also investigates his hidden tree house, where he laid his plans, and indicated fake locations on a map of London so as to send his pursuers in the wrong direction, including (coincidentally), the docks where Linthorn jumped Enola. She finds flower pressings in a travel book, and theorizes where the marquis is actually hiding. She is surprised by Tewkesbury's grandmother, the Dowager, who speaks with 'May' regarding the boy's disappearance, and tells her about his father. She says he was a "new thinker", and young Tewkesbury is likely the same. She tells 'May' to send her love to her grandson should she see him before she does, and lets her leave without trouble for snooping.Sherlock follows clues from Ferndell to Edith's tea shop, and asks to speak with her, bargaining for her time with the leverage of her banned books she lets her customers read. He inquires as to Eudoria's whereabouts, saying she needs to return to take care of Enola. Edith tells him Eudoria is doing good work to change the world, and Enola seems to be taking care of herself. Edith also tells him of the "useless boy" Enola ran into, and that she is also looking for Eudoria. She admits she is pleased that Sherlock is at least taking an interest, as Eudoria figured he would always ignore his younger sister. She says that Enola needs Sherlock, and advises him to find and help her. Later, Enola arrives at Covent Garden Market, where she tracks down Tewkesbury selling flowers among the crowded stalls. He is pleased, if surprised, to see her again, and asks why she went through the trouble of finding him, thinking she might turn him in for the reward money being offered. She tells him that she is actually there because his life is still in danger, and they both confess to having missed each other since their departure. Enola takes him back to her lodgings house, where Tewkesbury finds a pile of newspapers. Enola explains how her mother left her, and she is waiting for a response to the message she sent her. They bond over both having lost their fathers, and how the rest of the people in their lives don't seem to want them. Enola tells Tewkesbury that despite his family being wealthy enough to send a detective to find him, someone in his family sent a murderer instead. Enola says that she is sure his father was actually murdered, and that whoever did it must want him dead for the same reason. She also informs him of her findings at Basilweather, and that the branch that nearly killed him had been intentionally cut. Lestrade ambushes them, but Enola notices him and counters. They manage to make it to Enola's room, but struggle to keep the door closed. Enola tells Tewkesbury to go out the window and disappear, as his life will be in jeopardy if he is caught, and there is not enough time for them both to escape. Tewkesbury argues, but Enola orders him to go, holding the door just long enough for him to get away, and is caught by Lestrade . She is turned over to Mycroft, who confiscates the money her mother gave her, and takes her to Miss Harrison's Finishing School for Young Ladies. She is kept locked in her room in between lessons, and tries to remain defiant of the headmistress, though her spirits begin to dwindle. Later, Enola is told that her brother has come to visit her, and she is relieved to see that it is Sherlock. She checks his newspaper for a possible message, and is angry about her situation. Sherlock encourages her to take the lessons to heart, as they may come in handy in future cases. She asks him about Eudoria, and he says he has tracked her trail (albeit by following separate clues) and found the docks one step behind Enola's confrontation with Linthorn, and they discuss the bombs found there. She accuses him of helping Mycroft catch her, and seeing her only as a case, which he denies, and admits that he came because he cares about her. He asks her about the Tewkesbury case, having deduced that she was with the boy on the train. He is impressed with her detective work, he asks if she has solved the mystery yet. He gives her some advice, detective to detective, and also gives her Dash, the pine cone dog, having found it among Eudoria's things, telling Enola that he and Eudoria both find her extraordinary. After Sherlock leaves, Enola looks through the paper, and finds that the Reform Bill is still tied in the House of Lords, and another article about the still missing marquis. Just then a large basket is delivered to Enola, supposedly from Mycroft, but is revealed to be Tewkesbury, having smuggled himself in. Enola is overjoyed to see him, and is relieved that he is alright. He intends to get her out the same way he came in, but Enola alters the plan to get around Miss Harrison's watchful eye. Tewkesbury delivers the basket with Enola inside to Miss Harrison, telling her it's from Mycroft (who she is sweet on), and Enola escapes while Miss Harrison sends the lad away. They steal her motorcar, and Enola tells Tewkesbury that he is being targeted to prevent him from taking his seat in the House of Lords and voting yes on the Reform Bill, and that his uncle (who would inherit the seat and vote no) has the most to gain by killing both his father and him. Inspired by Sherlock's advice, Enola decides to go to Basilweather and confront Sir Whimbrel. They enter, but find the house dark and deserted, and are trapped inside by Linthorn, who was waiting for them and is wielding a shotgun. Enola and the marquis hide in the shadows, and narrowly avoid being shot. Enola jumps the gunman, but is rendered unconscious. Before Linthorn can kill her, Tewkesbury attacks him, knocking his gun away, but is also subdued. Linthorn begins to strangle Tewkesbury as Enola struggles to regain consciousness. She manages to grapple Linthorn, knocking him headfirst into a sharp fixture, killing him. The Dowager enters, and Tewkesbury is heartbroken to discover that it was she who hired Linthorn to kill him and his father. She picks up the shotgun, telling her grandson that she had to prevent him from voting on the Reform Bill. She tells him that she sent his mother and uncle back to London to look for him, and explains that the future of the country depends on his death. She shoots Tewkesbury in the chest in cold blood, seemingly killing him. Enola disarms the Dowager, and mourns over Tewkesbury. However, Tewkesbury wakes up, having only been knocked out from the impact, and reveals that he hid a steel chest plate from one of the suits of armor under his jacket, protecting him from the shot. He rises and tells the Dowager her time is over.Sherlock arrives at Scotland Yard and tells Lestrade to arrest the Dowager, as she is trying to kill the marquis, and had his father murdered. He explains that Sir Whimbrel was away at war and couldn't have done it, but that the Dowager wanted him to stand against the Reform Bill and any other such changes, as the Viscount and his father before him would have voted in favor. Lestrade reveals that Enola has actually solved the case first, impressing Sherlock. Tewkesbury reunites with his mother and uncle, and takes his seat in the House of Lords. Enola sees him off at the gate. They share an emotional farewell, but promise to see each other again soon, and Tewkesbury prepares himself for the final Reform Bill vote. Enola discovers a coded message for her in the paper asking to rendezvous, but upon decoding it, surmises that Sherlock, not Eudoria, has sent it. She disguises herself and goes anyway. Sherlock is indeed waiting for her, but Mycroft says she's outsmarted him. Sherlock offers to take Enola in as his ward once they find her, which Mycroft agrees to, annoyed that with Tewkesbury's vote, the Reform Bill has passed. As they leave, Sherlock notices Dash, but doesn't see Enola, and we see her watch them as they go. She returns to her new boarding house to find her mother waiting for her. Eudoria explains that she wanted to tell Enola where she went, but that it isn't safe and she can't stay too long. She is proud of Enola for saving the marquis, and they briefly reunite before Eudoria leaves again, telling Enola to send her a message if she ever needs help. Enola tells the audience she will have to follow her own path, and that it's time she found her place in the world as a detective, saying that "Our future is up to us".