In the 15th century, Richard Duke of Gloucester, (Basil Rathbone) aided by his executioner Mord (Boris Karloff), eliminates those ahead of him in succession to the throne. After the death of King Edward IV (Ian Hunter), he becomes Richard III, King of England, and need only defeat the exiled Henry Tudor (Ralph Forbes) to retain power.

Having already made a royal presence in The Private Lives of Elizabeth and Essex, Vincent Price returns to the Tower of London to play King Edward IV's younger brother who is drowned in a butt of malmsey by the then horror king, Boris Karloff.
And, of course, the future horror king, Price, would send up that particular scene in Theatre of Blood as a comically bad Richard III 34 years later - having already played Richard in the tedious 1962 Corman flick Tower of London.
But here, he plays Clarence as an effete, whiney drunk. It's certainly not Skaespeare, but grand stuff indeed..