Age range 12 to 17

Someone is draining the Overworld oceans of resources . . . but to what end? It could be a ploy by the griefer army to draw Captain Rob and his cavalry members away from the United Biomes of the Overworld capital, which they risked their very identities to defend. Or it could be theft on a scale so grand that only the deepest of oceans can contain it.
At the same time, it seems no coincidence that priceless relics and one-of-a-kind gemstones go missing from the new Overworld museum. The spectacular monument to the history of the free world is a worth a fortune. Housing the valuables of the Overworld makes Capital City Museum a target for every rogue player—and for the Griefer Imperial Army, now led by a gang of villains spawned by Lady Craven. When key pieces of UBO heritage are stolen, the troopers of Battalion Zero must pool brains, brawn, and every bit of luck they have to recover them.
The six friends must pull off the perfect heist: Captain Rob, leader and delegator ... Corporal Frida, master of stealth and disguise . . . Quartermaster Jools, strategist and techie extraordinaire . . . Sergeant at Arms Turner, muscle personified ... Artilleryman Stormie, terrain expert . . . and Master of Horses Kim, the "bronc whisperer" all work together to recover the UBO's secret treasures in this tempestuous fourth book in the Defenders of the Overworld series.