After 20 years of grooming, Cosimo de' Medici replaces his late father in the Signoria of Florence, where he must decide whether to support a call for war.

As the war with Milan takes a heavy toll on Florence, Cosimo boosts morale and the economy by undertaking the impossible: building the cathedral dome.

While Albizzi uses the plague to turn Florentines against Cosimo and his dome, two Medici family members are lost. Marco forces a shocking confession.

After Cosimo's arrest, his wife tries to buy a Signoria member's help. Piero seeks to refute the charges, while Lorenzo takes a more drastic approach.

In exile in Venice, Cosimo finds himself drawn to a slave woman. Meanwhile, back in Florence, Contessina runs into her childhood sweetheart.

Cosimo returns to Florence while Rinaldo is held for trial on treason and tyranny. Cosimo is visited by an unexpected guest: the pope. Cosimo convinces the Signoria to exile Rinaldo.