The team explores a mystery close to home as a cave linked to Warrior Queen Boudica is discovered near their UK base and Gillian and Ben find themselves in a race against time to escape dangerous gases when they're trapped underground. When a dig at the famous Roman Baths unearths a beautiful and ornate cave, it soon becomes clear that it was used to hold two prisoners. Team leader Gillian is convinced that Boudica wasn't killed in battle as believed, but was actually held captive for many years – could this be the place where she was hidden away? But who was the other one and what was their connection to the Warrior Queen? Unbeknown to the rest of the team, Gillian and Ben go back inside but are trapped as a tunnel collapses, and have to dodge centuries-old booby traps and toxic gases in their desperate bid to escape.

The team travels to western France to get to the bottom of a mystery that originally seems to date back to the First World War, but actually has its roots even further back in history. When a British tank inscribed with the name Joan is discovered underground, it looks like its occupants were murdered in a horrific war crime. But as the Bonekickers delve further into their research they find that, although the tank is British, the dead soldiers are both British and German. Gillian gets news that her mother is seriously ill, and despite the best efforts of Vivian and the rest of the team, refuses to leave the dig. The team uncovers a daring conspiracy involving two sets of enemy soldiers whose shared passion for the past which lead them to try and work together for peace in the midst of major conflict, a plan that could have changed the course of European history.