Young sleuth and artist, Hal, is on his most dangerous adventure yet in this sixth standalone book in the Adventures on Trains middle grade series! All aboard for Hal's most terrifying adventure! Harrison "Hal" Beck has been on countless adventures with Nat, his travel journalist uncle, and filled countless sketchbooks with his crime-solving art. This time, they're off to travel the night train to see the northern lights in the Arctic--a Christmas present from Uncle Nat to Hal that's supposed to be a fun vacation. But when Hal joins his uncle in Sweden, their tourist plans are quickly derailed. Uncle Nat's friend has just been awarded a Nobel Prize so someone broke into her house in search of a "Kill Code" sound frequency they think she has. It's a frequency that has the power to kill many people, so it can not fall into the wrong hands. And are Hal and his uncle being followed? As if that isn't worrying enough, a man from Uncle Nat's past has resurfaced: someone thought to be dead who holds a grudge against Uncle Nat for the role Nat played in his attempted murder. With an assassin on the loose, a potential stalker on their trail, and the "Kill Code" being searched for, Hal is in for his scariest and most dangerous ride ever!.