Historians and researchers chase down two of the medieval and early modern world's most stubborn cold cases. One thread follows the legend of Ivan the Terrible's lost library, a vanished collection said to hold ancient Greek and Roman texts smuggled into Russia through the Tsar's court, with experts weighing centuries of failed searches beneath Moscow against the possibility the library never existed at all. Another traces the fate of Joan of Arc's body after her execution in Rouen, following the trail of relics, burial claims, and forensic examinations that have tried and failed to confirm what became of her remains. Presenters and specialists move between archives, excavation sites, and expert interviews, laying out the documentary evidence for each mystery alongside the theories that have grown up around the gaps in it. Rather than offering a tidy solution, the film treats both cases as open files, showing what has actually been found, what has been claimed without proof, and why each mystery has kept historians arguing for centuries.