The 'Valkyrie' in Germany refers to the 20 July Plot in 1944. A group of German officers attempted to assassinate Hitler. They named their operation 'Valkyrie'. Their motivation was mainly due to the fact that they saw Hitler leading Germany to destruction in World War II. They wanted to take control after his death and negotiate a peace with the Allies.
The 'valkyrie true story' is about a significant event in World War II. It involves a plot. Well, it was a plot by some German military officers to kill Hitler. Valkyrie was sort of like the code name for their operation. These officers were aware of the evil that Hitler was doing and wanted to end the war. They thought by getting rid of Hitler, they could make peace with the Allies. But, as you know, the plan didn't work out as they hoped.