A gold shipment being transferred to payoff some of the US debt to China is stolen from an armored truck by a group of costume-wearing thieves. The clown is found dead in a dumpster. Through his costume, they track down one of his associates. When they get to his house, some of the other thieves are found shot to death on the floor. They eventually track down the gold shipment, but the gold is fake. It's gold-coated tungsten.
They determine that the theft is not just about the money and value of the gold, rather it is an attempt to destabilize the US economy through the gold reserves. The ringleader has ties to a terrorist organization. They track down the fake company that is producing the tungsten filled gold bars through a massive tungsten order. NCIS shows up and takes down the operation, which has ties to Iran.
The theft and operation must remain a secret to protect the integrity of the US economy.