Buckle up: this one’s complicated.
An impatient Englishman named Slade checks in to a remote hotel in the French countryside. Exhausted by his journey through a snowy night, he falls asleep fully clothed on the bed of room 18. He wakes to find himself being handcuffed to the bed by Eliza.
Spluttering, he demands an explanation. He is the legendary conman Charles Percival, Eliza declares, and she has detained him on behalf of a client defrauded by him when he posed as a bookmaker at the Grand National horse race in England. She has notified the police and awaits extradition to bring him back to England, which likely won’t happen until the next day.
The man insists that Eliza is wrong: he is a solicitor by the name of Jeremiah Slade, in France on confidential business. When he calls for help, she gags him. Eventually, she lets him speak again and explains her evidence. Percival has been wanted for twelve years, but always adopts a different character and has never been caught. However, she managed to track a journey leading her to this hotel, with all stages of the travel booked under the name Jeremiah Slade.
The bound man once again cries out—and this time a hotel employee hears. He barges in and responds to Eliza’s explanations by asking if she is working with the other English detective.
Who is the other detective but Patrick Nash, Eliza’s rival and would-be employer. He immediately surmises Eliza’s reason for being at the hotel—because it’s the same as his. But Nash has beat Eliza to catching Percival: he has the conman handcuffed in room 6.
This man also denies all accusations. He is Sebastian Baron, from a well-connected family. Eliza simply smirks and leaves Nash, to his surprise. She asks the hotel employee not to tell Nash about her own captive.
Unfortunately, Nash is waiting in room 18 when Eliza returns. He saw her arrive at the hotel and enter that room. He convinces her to unshackle Slade as an enticement to make him talk. Slade explains that he is at the hotel on behalf of an English client who lives in the area with his local wife, but the marriage is now ending and the client needs legal help.
Nash brings up the arrest last year of a man the police thought was Percival, but was in fact just an accountant to whom the real Percival had created a false trail of incriminating clues—a cautionary tale. Eliza insists she has the right man, and convinces Nash to let her talk to Baron.
Baron says he is traveling through Europe before taking up his late father’s estate in England. Nash explains that he knows Baron is actually Percival because he found a safe house paid for by the fake bookmaker at Grand National; inside were travel documents for Baron leading Nash to the French hotel.
Eliza is perplexed by one thing: if either she or Nash is right about one of their captives being Percival, why would he set a false trail leading to his actual location, instead of misdirecting followers to somewhere far afield? She and Nash decide to question their suspects together.
Before they can get much further than the two men accusing each other, Eliza is called down to the lobby: the local police chief has arrived, to work on Percival’s extradition. The chief is appalled by the situation and demands that Eliza and Nash wait outside while he questions their captives himself.
After having a drink, the two detectives decide to check on the police chief—and find him on the ground, dead. Baron says he asked for water and the chief drank some and collapsed. Baron fears that he too will die, claiming to have also drunk—but Slade says that’s a lie: Baron is actually the poisoner, and is trying to kill Slade, because Slade is Percival.
In a separate room, Slade tells Eliza that he believes Baron is a paid assassin—he saw Baron waiting in Slade’s London home with a gun recently. One of the wealthy people defrauded by Percival—Slade, supposedly—at the horse race was a Russian gang leader, who might be trying to exact revenge.
Meanwhile, Baron tells Nash that he’s Percival, while choking and calling for a doctor because of the poison he supposedly ingested. Nash goes to loosen Baron’s collar—and Baron grabs Nash’s gun. He makes Nash release him, then gathers Eliza from the other room and handcuffs both detectives in the cellar of the hotel. He explains that he’s not Percival, but an American detective sent to find the fraudster. He also found Percival’s safe house in London and waited there, but had to flee when Nash’s men arrived. No matter; now he has Percival, aka Slade, in his grasp, and will be taking him in to the police.
Stuck in the cellar, Nash admits that his firm is overstretched and that he only found his way to France because he broke into Eliza’s office and saw the case tacked up on her wall. He talks about his brother, with whom he started the firm, and how he was not just a good detective but a good man. He died trying to help an immigrant Irish family, gratis.
Eliza doesn’t believe a word he says—but he does get free, using his tie pin as a lock pick on his handcuffs at her suggestion. He shares the pin with her to do the same, and then they head back upstairs to learn that a horse and carriage has been stolen—presumably by “Baron” to bring Slade to the police.
Eliza and Nash part ways, talking to different guests at the bar. Eliza learns from a visitor that he’s visiting at the suggestion of Slade, who is trying to sell the hotel on behalf of the owner, a man who was married to a local girl and is now divorcing her. But Slade and the owner never turned up for the dinner, so the visitor tried to leave—but a snowstorm has blocked the only road out for the night.
Eliza interrupts Nash’s conversation with an alluring woman named Kinsky to tell him that “Baron” and Slade must still be at the hotel, stuck because of the snow. They find “Baron” chained up in room 6; he says someone must have knocked him unconscious. Nash retrieves his gun and they head to room 18, which is empty.
But Eliza has a realization: the now-dead police chief wasn’t actually the police chief; his coat was dry when he arrived, despite the snowstorm, so he was at the hotel the whole time. Searching the guest ledger, the two detectives find a room booked under the name of the winning horse of the Grand National.
There they find Kinsky, the woman from the bar, about to poison Slade and the hotel employee, who are tied up. Nash stops her by brandishing his gun. She is the assassin hired by the vengeful Russian gang leader; she poisoned the water that killed the “police chief.” And both Slade and the hotel employee—as well as the dead “police chief”—are Percival. They all worked together on cons.
The hotel employee is the one who knocked “Baron” out, saving Slade. And he introduced his other associate as the police chief to Eliza in order to save Slade from her. That now-dead associate was to play the role of the hotel owner alongside Slade and sell the hotel to the wealthy visitor, then pocket the money. The real hotel owner is indeed being divorced and is preoccupied by drinking his sorrows away in the nearest town. He let the conman who was pretending to be an employee essentially take over the hotel in his absence.
All is cleared up, and the criminals apprehended. When the roads clear in the morning, Eliza and Nash head into town to breakfast—together.