It is the spring of 1918. After four years of pointlessly bloody stalemate, the German Army is once again marching on Paris. Inside the City of Lights, soldiers and civilians struggle to find some semblance of a normal life.
At a clandestine meeting at St Sebastian Metro station, MI6 agent Neville Wordsworth and Office of Naval Intelligence agent Robert F. Scott meet to discuss the handover of a "liberated" Da Vinci work. The painting was secured by Wordsworth from the previously German-held chateau of Comte Francois Bourgine. Intelligence on the work's whereabouts was provided to Wordsworth by Scott, which Scott gleaned from meetings with the Comte's daughter, Camille. The painting is to be handed over to American authorities in exchange for certain offensive maneuvers to be undertaken by the American Expeditionary Force. The drop is to take place later today at an anti-war demonstration. Unbeknownst to the two agents, they have been followed to their meeting…
Sitting across from the pair sits Camille Bourgine, traveling incognito, and budding journalist Eliza 'Jax' Olney. Olney met Neville Wordsworth when he was an REMF on the Western Front. Since then, she has been working on a story about him, as he has recently become heavy involved in the anti-war movement. The story is shaping up to be the greatest piece about the War ever written. Camille knows more about Wordsworth's activities than Jax does however. Recently she was in a relationship with Robert F. Scott, who has been posing as the French actor Pierre Depardieu. When it became apparent that Depardieu was only using her for information on her father's collection, Camille tried to go public with the information. Scott pulled his strings to make her accusations seem like the hysteric ravings of a childish schoolgirl. Camille knows the Scott-Wordsworth connection and believes that with Jax's help she can reveal the full story, clear her name, and restore her family's collection. Jax eats up this information: the greatest piece about the War ever is getting better.
Jax leaves Camille, and Neville leaves Scott. Scott spots Camille leaving the station and confronts her. Scott is all tough guy bravado but melts when he see's how much he's hurt her: is America's manifest destiny worth breaking the heart of this innocent French girl? They agree to meet up later to try to smooth things over. Deep down, Scott really cares for her.
Neville and Jax make their arranged brunch meet. Jax reveals she knows about the painting, and accuses Neville of stealing the painting to profit personally from the war. Neville swears that it'll end the war quicker if the painting is handed over to the Americans, which is a better outcome for everyone. Jax needs to know the significance of the painting, but Neville is unaware. He knows some questions are best left unanswered. They agree to meet later at the demo.
While sipping her coffee, Jax spots Camille and Scott walking cross the square to the Paris Opera. She follows them through the stage entrance and eavesdrops on the conversation. Scott explains that the painting is of the utmost importance to the US government, and bringing them stateside will win the war within months. When pressed, Scott states that his orders come from Project Testimony, somewhere in the deepest, darkest pits of the Office of Naval Intelligence, and that he doesn't know any more. At this point, Jax accidentally knocks over a dressing mannequin, and having being spotted by Scott and Camille, makes a hasty exit.
Back at her hotel room, Jax chain smokes as she furiously punches out the story on her typewriter. A loud crash is heard and three huge gendarmes kick in the door to her room. Cigarette still in mouth, Jax argues with the men, calling them to task to press charges or leave. They smash her typewriter and confiscate the story she is writing. Fuming, she packs her things and heads to the demonstration.
A riot of sound and colour marches its way through Paris as the anti-war protest gathers. One of the key speakers will be Neville Wordsworth. Jax eagerly awaits the speeches. Camille waits in the crowd too, while Scott watches the procession from a distance. As Neville begins to speak, Scott makes his way forwards to retrieve the painting which has been hidden under the stage. Jax spots Scott duck under the stage and follows, while Camille spots Jax and does the same.
Jax confronts Scott, she wants to know what the picture is. Scott refuses and Jax pulls out a military surplus grenade, threatening Scott with it. Camille begs the two to stop. A rousing speech is heard above the stage; the crowd cheers. Jax and Scott argue loudly; some claptrap about national security from Scott, falling on the deaf ears of Jax who needs to understand the mystery. Neville hears familiar voices from under the stage and heads under. He quickly tries to diffuse the situation which has gone far above everyone's heads. In the heat of the moment, Jax professes her love for Neville. With this, a scream is heard from a civilian at the sight of the grenade. Shouts of "terroriste!" can be heard from the crowd as the gendarme move in and arrest the group.
Wordsworth and Scott are able to get the group's names cleared. Time passes. The war ends in a bitter armistice.
Camille returns to her ancestral home but finds the place hollow after her adventures in Paris. She decides the New World is where she needs to be be and crosses the Atlantic to take up residence in New York. Working odd jobs, she finds she has a real knack for solving mysteries, especially those with an occult bent. She starts the First Ladies Paraphysical Detective Agency, and develops a decent reputation amongst certain eccentrics in New York. Wordsworth returns to England a knight and sets up a family estate in Oxfordshire. Scott returns to New York to work for the Department of Treasury as an Agent. After spending some time with Neville, Jax begins to get itchy and travels the world investigating blood cults. Now working under the pen name of Jackson Elias, she publishes treatises on cults as far and wide as the witches of England to the Thugees of India. Despite the fact that Neville moved to New York to be closer to her publishing house, their relationship becomes estranged, and the engagement never eventuates in marriage. Wordsworth pours himself into his American assets. Scott is hand picked by J. Edgar Hoover to be part of his Bureau of Investigation. Agents Scott and Cooper are tasked with investigating cult and ritual murders in New York. It is here that Scott once again crosses paths with Camille. The two build a friendship neither of which would have thought possible 7 years ago.
Suddenly Jackson Elias returns to New York…