On D-Day, Magnus and her team helped clinch a victory for the Allies that, unfortunately, involved the burial of a rare Abnormal called a Fire Elemental. Now, in Carentan, France, half a mile from where that Elemental plunged into the Earth, Ravi, the head of the Mumbai Sanctuary, has disappeared without a trace. Magnus, Declan, Kate and Will gather some supplies and head to France to find him.
But upon arrival, they find the area heavily guarded and are confronted by a military commander who refuses to reveal his name or national affiliation. Magnus and her team leave the premises, but find another way in: The tunnel dug in WWII by the Resistance leads them exactly where they want to go. Magnus and Will lead the charge, with Kate and Declan keeping watch.
But Will and Magnus don't get very far before an unseen force throws them to the ground. They soon find that they are in a time dilation field - a dome where time moves much more quickly. Ravi has been trapped there for what is, to him, several decades, but only a few days have passed outside the dome. If anyone tries to exit the dome, they are obliterated, so Will, Ravi and Magnus must attempt to dissolve the field from the inside.
Kate and Declan manage to steal a phone and get in touch with Biggie, who has received a highly condensed message from Magnus explaining where they are. When Declan attempts to persuade the commander that they can dissolve the field, he notices a satellite image of the field with a symbol - written in an assemblage of stones - for the Hollow Earth element cerillium. It's a message from Magnus! He and Kate rig a jeep, bearing an unarmed missile, to drive into the time field. The missile isn't cerillium, but it might do the trick.
Magnus has devised a node, similar to the one Adam Worth created, to dissolve the field and needs the cerillium substitute to complete it. She knows that, if she is successful, anyone inside the field - the descendents of the original scientists who attempted to solve the mystery - will cease to exist due to the space-time continuum breach. But, if she does not employ it, the field will expand to cover the Earth, and the acceleration of time will cause plates to shift and oceans to heave at a rate that would literally tear continents apart. It's either the dozens of residents of the field, or everyone on Earth.
The residents are aware of this, as well, and do everything they can to stop her. One woman fires a gun at Will just as Magnus activates the node. Ravi, having aged almost to the end of his life, throws himself in front of the bullet, saving Will's life but losing his own.
On the other side of the field, Magnus can't let the coincidence go: It's too strange that this time-space anomaly occurred so close to where she buried the Elemental. It looks like she has some work to do to get to the bottom of things.