Steel is the third act in the Age of Empires III campaign. Its main protagonist is Amelia Black.
It starts with Amelia Black overlooking the American railways, when she is approached by an old coot. She is being helped by Major Cooper and his Cavalry, who are protecting her men while they work. The two go to a nearby fort after claiming most of the railways, which is under attack by the Mexican Army. With the help of Beaumont and some heavy artillery, Amelia and Cooper are able to drive away the Mexican Army. Beaumont asks Amelia to go up north for a new mine, which she accepts.
After following Beaumont into the mines, she is approached by her Granduncle, Ká:nien. He then warns her that Beaumont means harm. In fear of being attacked by Major Cooper's Cavalry, Beaumont scrambles into the mines. Amelia and her men explore the local area, finding several tribes desperate for food as Beaumont had caused their sources of food to deplete. They find several friendly soldiers and natives trapped in stockades, along with treasure related to the Circle and Food Wagons, which they send to the tribes in return for warriors. They reach the end of the system where Cooper sends several men to capture Beaumont, but are killed by him. Ká:nien then tells Amelia that Beaumont is the new leader of the Circle of Ossus and that he wanted to find out if she knew what happened to the Fountain of Youth's water. Ká:nien then tells Amelia of the lost Spanish gold in the swamp where the Lake of the Moon once stood.
The trio then travels to Florida to take the Spanish gold from the Circle of Ossus. They are successful and also find the old Fixed Gun that Morgan had used to destroy the Fountain of Youth. However, Cooper is killed after he confronts Beaumont, who sends his Pure White Wolves at him. Amelia buries Cooper and asks the Seminoles what had happened to the water. They tell her that the Incans took some water after Morgan destroyed the Fountain.
She then sets sail for South America in hope of finding the Last City of the Inca. She meets with Simón Bolívar, who is attempting to lead a revolution against the Spanish. After liberating several towns and destroying the Spanish command post, Simon is grateful for her services and tells her how to get to the Incan city. Amelia and a few of her men travel through the Andes Mountains, taking shelter from the bitter winds outside. Unfortunately, Beaumont had hired a group of Black Riders to kill her before she reaches the Last City of the Inca. Amelia and her men take down the numerous scouts sent to disclose their location to the group, and eventually reach the Incan City.
The Incan City is under attack by the Circle, knowing that they have barrels of the fabled water. Amelia and Ká:nien, with the help of the Incans, chase out the Circle. However, Beaumont managed to steal several barrels of the water before they were chased out.
In fear that the water does hold the key to immortality, Amelia and Ká:nien chase Beaumont to the Circle's stronghold in Cuba, the Ossuary. They are ambushed by several Circle members on the way to Havana, but manage to defeat them. After reaching Havana, the Cubans want the Circle gone and help Amelia set up a base to attack the Circle. With help from the American Navy and the Cuban Army, Amelia and Ká:nien destroy the Ossuary and check if Beaumont is really dead. Beaumont rises up and attempts to stab Amelia, but she grabs her rifle and kills him before he is able to. Her men later take the Spanish gold the Circle had taken from Florida and share it among themselves.
Amelia returns to America to once again encounter the old coot, he then congratulates her on finishing the railway and saving the company. He then says, "You broke the Circle, and in just one lifetime too." It is then revealed that the 'Old Coot' was actually her Great-Great-Grandfather. He then says "Don't miss your train", and then walks away.