It's the Second World War. The Red Army had to retreat in a hurry and left their top secret weapon, "Katyusha", behind. A group of 8 soldiers is sent to do everything possible not to let "Katyusha" get into the hands of the Germans.
It was the rockets of the Katyusha that were kept top secret, not the launching rails. The Russian propellant was more efficient than similar German rockets. This movie only has launching rails on the truck, no rockets.