This is one of a series of eleven historical pictures, painted by Opie between 1792-1800, for the illustration of Bowyer's edition of Hume's History of England, published in 1806.
Boadicea was the wife of Prasutagus, the King of the Iceni, a powerful and warlike race. Subjugated by the Romans, when Prasutagus died he bequethed his entire property, including his family, to the Roman Emperor in an attempt to secure them from harrassment. However, the Romans merely used this as a pretext to prey upon them, and in a desperate response to their treatment Boadicea headed a revolt. The picture depicts Boadicea addressing her fellow Britons urging them to rise up against the Romans. After initial successes, Boadicea and her troops were defeated, with the devastated Queen taking her life by poison.