Was Peter III a finicky man-child with a violent streak? Probably not, but The Great certainly wasn't far off the mark in depicting the emperor's unpopularity, as Montefiore describes. Peter came to the throne in 1762, following the death of his aunt, the Empress Elizaveta (we'll get to her in a moment). He was the grandson of the popular Peter the Great but had been raised largely by his German family and was strongly pro-German in his policies. He only reigned for about six months before his wife, Catherine, deposed him in a coup.
Peter was unpopular, but The Great does exaggerate some historical details to make him a delightfully monstrous antagonist. He did attempt to make movements toward modernising the Russian military and abolished the infamously violent branch of secret police. He also was a proponent of education and religious freedom. Still, his taste for brutal "jokes" and "pranks," as well as his self-proclaimed temper, are historical record as well. He was deposed and ultimately died soon after the coup; whether he died in a drunken brawl or was outright assassinated remains a question among historians.