The Stone Mask (石仮面 Ishi Kamen) is the third story arc in Phantom Blood.
After leaving his father George Joestar I in the care of trustworthy physicians, Jonathan leaves for London to analyze Dio's poison in hopes there is an antidote to it. The moment Jonathan leaves, Dio breaks into his adopted brother's study to steal the Stone Mask and decides to arrange an "accident" for Jonathan involving the mask, which is presumed to be a form of torture device. But when Dio learns that Jonathan's search brought him to the treacherous rookery of Ogre Street, he expects some mugger to murder Jonathan in his stead.
While walking along the maze-like Ogre Street, Jonathan is set upon by a trio of muggers with their leader giving his friend Tattoo the honor to gut their victim. Determined by his cause, Jonathan risks injury by grabbing the blade of Tattoo's knife while taking him and the Chinese man out. Jonathan then easily knocks down the third mugger with a kick. Impressed by Jonathan's show of resolve and mercy, the mugger stops a whole band of thugs from attacking the gentleman. When Jonathan explains his reason for coming to Ogre Street, amused by his naivety and nobility, the mugger introduces himself Robert E. O. Speedwagon and offers to take him to the apothecary he is looking for.
Meanwhile, anxious if Jonathan has been killed or has foiled his plans, Dio wanders the streets while hating himself for drinking like his father had. Dio eventually bumps into two vagrants who taunt him for being out so late at night. When one of the drunkards mentions Dio's mother, Dio decides to test the Stone Mask out as he smashes his wine bottle into the drunkard's face. Dio then takes one of the vagrants' knife and slits the first vagrant's throat, subjects the other vagrant to the Stone Mask. The mask activates, and Dio perceives a bright light bursting forth from the Stone Mask as its prongs pierce the man's skull before he collapses. Before Dio leaves in disappointment over such an anticlimactic end, the vagrant rises up and assaults Dio, revealed to have developed a set of monstrously long canines. The inhuman monster overpowers Dio with his superhuman strength before grabbing Dio by the neck when he attempts to escape into the river. Feeling the life being drained out of him, Dio nearly dies as he is saved at the last moment by morning sunlight which mysteriously disintegrates the monster.
Dio arrives home to find Jonathan waiting for him, and learns that he has already cured his father and alerted the authorities. When Dio feigns remorse, Speedwagon arrives to warn his friend not to believe anything Dio says. Speedwagon then pulls Wang Chan from behind the curtain, followed by a saddened George and a group of constables whom are to take Dio in for attempted murder. When George leaves, not wanting to watch his adoptive son getting arrested, he is troubled when Wang Chan mentions having read Dio's fortune that he has the devil's luck this night.
As Jonathan proceeds to place the cuffs on him at his request, with nothing left to lose, Dio explains that his downfall was the result of being human. As Jonathan expresses confusion over the seemingly defeated man saying that he now forsakes his humanity, Dio reveals the knife and Stone Mask on his person and attempts to stab Jonathan. But George shields Jonathan and is stabbed in his place. As a dying George collapses in Jonathan's arms, Dio adorns the mask and splatter blood over it, commencing his transformation. The constables then all shoot at Dio, sending him crashing through a window.