The Ring have used the mask to store contraband material to smuggle it into the country, as antiquities do not require the same screening process as regular cargo. Chuck, Sarah and Casey replace the mask with a replica in order to retrieve its contents.
Once Team Bartowski successfully steal the mask and replace it with a replica, Sarah and Shaw examine the mask at Castle, hoping to reveal its secrets, while doing so the cause a canister to drop out of the display base, deploying toxic cyclosarin gas, a designer weapon developed for battlefield use. There is a counter-agent that can be taken up to 30 minutes after exposure. After Shaw and Sarah are poisoned, Chuck must obtain the counter agent from the museum where it is concealed in a stolen antique vase.
The mask depicted in the episode is actually a poor copy of the Mask of Agamemnon, an actual gold funeral mask discovered at the ancient Greek site of Mycenae. The real mask is on display in the National Archaeological Museum of Athens.