A small group of American soldiers find horror behind enemy lines on the eve of D-Day.
On the eve of D-Day, the 5th of June, 1944, several American paratroopers are dropped behind enemy lines to carry out a mission crucial to the invasion's success: destroy a radio tower built in a little castle of an old French town that the Third Reich uses for communication between Berlin and Normandy beaches' bunkers. Due to the intense enemy fire, the planes are shot down and most soldiers die in the landing or are killed by the Nazis' night patrols after they land. However, a private named Ed Boyce survives to find Corporal Ford, a last-minute incorporation from Italy and a veteran expert in bombs and explosives, rogue sniper Tibbet, war photographer Chase, and finally private Dawson. After they watch the killing of their superior Sargeant Eldson by a Nazi night patrol, Ford becomes leader of the group and they try to get the town with the tower in order to complete the mission. In the forest close to the town they meet Chloe, a villager who assists the soldiers in her home with her elderly aunt and little brother Paul. But the mission gets complicated when, by accident, Boyce ends up inside the Nazi radio tower, only to find horrible secret experiments with human beings that could empower a victory for Hitler and would change the fate of the world forever. With D-Day approaching, Boyce and the others begin to realize there is more going on in this Nazi-occupied village than a simple military operation. They not only have to fight against soldiers to destroy the radio tower, but also against the results of the experiments to turn the entire place to ruins. All this before dawn and Hitler gets his Third Reich of a thousand years that he promised.
On the eve of D-Day, a paratrooper squad is sent to destroy a German radio tower in an old church; however, their plane is shot down before they can reach their target, leaving only five apparent survivors, Corporal Ford and soldiers Boyce, Tibbet, Chase, and Dawson. Dawson is killed by a landmine but the team continues onward, running into a French woman named Chloe who agrees to take them into the village where the radio tower is located. The soldiers learn that Chloe lives with her 8-year-old brother Paul and her aunt, who has been disfigured by Nazi experiments taking place in the church. After Tibbet and Chase depart to check the scheduled rendezvous site, the remaining soldiers are forced to hide in the attic when a Nazi patrol performs a routine inspection. But when SS Hauptsturmführer Wafner attempts to rape Chloe, Boyce attacks him, forcing the soldiers to take him prisoner..
A team of paratroopers led by Sergeant Eldson (Bokeen Woodbine) are heading into Germany with orders to take down a Nazi radio tower from a church. Just as they near their target, their plane is shot at by enemy soldiers. The surviving men jump from their plane before it explodes, and they land in the water before swimming to land.
Private Ed Boyce (Jovan Adepo) reunites with Corporal Ford (Wyatt Russell). Boyce spots Eldson being found by a group of Nazi soldiers and sees as they execute him. Ford keeps Boyce quiet as they move and find Tibbet (John Magaro), Chase (Iain De Caestecker), and Dawson (Jacob Anderson). They walk toward their mark, and Dawson accidentally steps on a land mine that blows him up. The others keep moving into the woods as they spot a French woman named Chloe (Mathilde Ollivier) walking by. Ford orders them to run after her since nobody can know they are there. The men catch up to her, and Boyce speaks to her in French to let her know they mean no harm. Chloe reluctantly lets the men follow her back to her home.
Chloe leads the soldiers into her village when a neighbor hears her and calls to the German soldiers for her being out past her curfew. Chloe talks to the Germans while the American soldiers stay hidden. She brings them back to her house where she lives with her kid brother Paul (Gianny Taufer) and Aunt Simone (Eva Magyar), who is terribly ill and is heard making an unpleasant and loud wheezing sound. The men hide up in the attic. Paul wants to play catch with Tibbet, but he coldly brushes him off before he and Chase go check out the rendezvous site. Outside, German soldiers take a man and woman outside and execute them. Chloe says this is what happened to her own parents.
Chloe tends to Boyce's bullet wound as she explains what happened to her family. Boyce also plays with Paul a little bit. Moments later, more German soldiers arrive for an inspection. They are led by Captain Wafner (Pilou Absaek). Boyce has to hide again as Chloe lets him enter her home. Wafner attempts to rape Chloe, but Boyce comes out and attacks Wafner, and the soldiers keep him tied up in the attic.
Ford sends Boyce out to find the rendezvous point. He comes across the church where the tower is located, but he is nearly spotted and is forced to hide in a truck filled with corpses of the other paratroopers. Boyce finds himself inside the church, and he comes across a laboratory where he sees a number of horrific discoveries, such as the head of a woman separated from her body but still speaking to him, as well as a bag with a barely living thing inside. Boyce hides and sees the mastermind, Dr. Schmidt (Erich Redman), entering. Boyce finds a fellow paratrooper named Rosenfeld (Dominic Applewhite) and pulls a massive needle out of him to save him. Boyce also takes with him a needle containing some kind of experimental serum before he and Rosenfeld flee.
Boyce brings Rosenfeld back to the attic and shows Ford the serum. He questions Wafner as to what it does, but he refuses to say anything, so Ford starts beating the crap out of him. Boyce explains to Ford what he saw in the church, so Ford starts to devise a proper plan to make their way in and take the tower down.
Ford orders Chase to go upstairs to untie Wafner so that he can get them inside. Wafner then attacks Chase and grabs his gun, shooting him repeatedly. Everyone runs upstairs to subdue Wafner, but while they try to help Chase, he ends up dying. Boyce grabs the needle and injects Chase with the serum. He soon comes back to life, but very quickly, he displays signs of enhanced strength and disorientation. Chase then starts to mutate horrifically and become violent. He attacks his comrades, forcing Ford to shoot him repeatedly. This doesn't kill Chase, so Boyce has to smash his head to bits in order to kill him for good. Wafner says that is what the serum does and is essentially being developed by the Nazis to reign supreme for a thousand years. Wafner then manages to escape as his comrades are outside, and he kidnaps Paul and gives him to the soldiers. Ford and his men fire at the Germans, and Wafner gets half his face shot off as he gets out. Ford then decides they have to take down the tower immediately, with Paul's rescue being second priority. Boyce and Chloe don't like this, so they plan to rescue Paul first.
Wafner returns to the lab and finds Schmidt. To Schmidt's horror, Wafner takes two needles with serum and injects himself, causing a rapid mutation and making him grow increasingly mad.
The Americans use a German soldier that they strapped with explosives to make their way into the church. Tibbet and Rosenfeld stay outside killing soldiers while Boyce, Ford, and Chloe head inside. Chloe is almost attacked by a soldier who tries to get her killed by one of the mutated people. The mutant kills the soldier and goes after Chloe, but she burns him to death with a flamethrower. She manages to get Paul out, but they still come under fire by the Germans.
Tibbet manages to get Paul through the gunfire, but is wounded himself. Meanwhile, Boyce and Ford encounter Schmidt and the mutated Wafner, who attacks them. Boyce fights Schmidt as he tries to stab him, but Boyce manages to kill Schmidt. Wafner then gets Ford and impales him on a meat hook. Boyce distracts Wafner as Ford frees himself and injects himself with the serum to fight Wafner. Before Wafner can get Ford, Boyce throws a tank and shoots it, causing an explosion. Boyce tries to help Ford out, but he chooses to stay behind and ensure that the laboratory is destroyed along with the tower. Ford gets an explosive ready as Wafner prepares to attack. Boyce runs out as Ford detonates the explosive, killing himself, Wafner, and every last mutant in there as the whole place goes down. Boyce reunites with the others outside and confirms that Ford is gone, while Chloe tends to Tibbet's bullet wound.
In London some time later, Boyce reports to his superiors on the mission, stating that Ford chose to detonate explosives from inside the tower to ensure that it went down. He also chooses not to mention a thing about the lab or the experiments. The superior then tells Boyce that he and his men will be reassigned.