At an old manor, the Scarecrow and Scream Queen are preparing for their latest scheme when Batman and the Flash arrive. The Scarecrow doses them with fear gas inside of a pumpkin and reveals he's rigged every pumpkin in the city the same way. The Flash disperses the gas and then runs off to get the pumpkins. The Scarecrow's skeleton-costumed men close in but Batman easily disposes of them. Scarecrow comes at him with a scythe.
Scream Queen goes after the Flash, firing bursts of sonic energy. he dodges them while recovering all of the pumpkins in town. Scream Queen gets close enough to blast him and then closes in. The Flash jams a pumpkin into her face, dosing her with fear and causing her to panic. Meanwhile, Batman defeats the Scarecrow and breaks his scythe. The Scarecrow makes a run for it only to discover the Flash has piled all of the pumpkins up at the door. They fall onto Scarecrow. The people arrive demanding to know who stole their pumpkins and the Batman makes a hasty exit.
In 19th century London, a coach arrives and a black-cowled man with a skull cane invites a woman inside. Nearby, the famous detective Sherlock Holmes and his comrade Watson are on a case when they hear a scream. They run to find the woman from the carriage, paralyzed from a spell. There have been similar cases recently and the locals think a stranger, Jason Blood, is responsible. They form a mob and approach Blood's house. Blood draws a circle with chalk and starts to utter an incantation. They interrupt him halfway through his transformation and place iron shackles on him. Holmes and Watson arrive but the locals haul away the demon.
Holmes goes over the clues and realizes that Blood is innocent based on the evidence. Holmes proceeds with the ritual that Blood was doing when he was interrupted.
Batman easily disposes of the villain Crazy Quilt in the middle of an art theft. A glowing circle appears and sweeps Batman back to the 19th century. Sherlock Holmes makes several accurate deductions about Batman's secret identity, and the Caped Crusader knows all about Holmes.
The locals are preparing to burn Blood outside and Batman drops down to rescue him. They escape to the rooftops and Blood insists that both he and the demon Etrigan are innocent. They rendezvous with Holmes, who says there have been eight victims and the last one was left with a note that refers to Etrigan and the fact there will be ten victims. They hear a scream and Batman and the transformed Etrigan investigate. They find the cloaked figure putting another woman in his carriage. Etrigan arrives first and the figure summons a giant serpent to battle the Demon. Batman arrives just as the figure completes his spell on the woman, using a mystic horn. Batman recognizes the figure as Gentleman Jim Craddock: the Gentleman Ghost before he died. Craddock transforms Batman's cape into a giant bat-demon that grabs Batman and lifts him up into the sky. Batman fires a line to a nearby tower and when they reach the end of the line, the bat-demon is brought to earth. Etrigan uses his fire breath to burn away the snake and then comes to Batman's aid. Batman's costume is ruined and Etrigan conjures him garb suitable to the period.
The heroes examine the comatose woman and Etrigan reverts to Blood as Holmes and Watson arrive. Batman explains Gentleman Jim's future. Batman and Blood go to Blood's home to check his laboratory, while Holmes goes off on his own and Watson gets the woman to safety.
At a windmill in the countryside, Gentleman Jim speaks with a demon, Asteroth, who is aware of Batman's presence.
Blood discovers the horn is connected to Asteroth, who Merlin and Etrigan confined centuries ago. They realize that Gentleman Jim will offer ten souls to Asteroth in return for immortality, and the energy will allow Asteroth to free himself. Watson arrives to tell them that Holmes has located Gentleman Jim's hideout.
At the windmill, Gentleman Jim attacks Holmes and the highwayman emerges triumphant. Batman arrives in time to stop him but Gentleman Jim uses a ring to summon a demon wielding a goremaul to battle the heroes. Gentleman Jim prepares to use the horn on Holmes and make him the tenth victim.
Etrigan advises Batman to remove the ring from the demon's nose to defeat it. They succeed but too late to stop Gentleman Jim, who departs for the Underworld with Holmes' stolen energy. Batman and Etrigan leap through the portal after him.
Gentleman Jim proceeds through the Underworld to where Asteroth is imprisoned. Etrigan throws Batman at him and then leaps across the pits only to find Asteroth waiting for him. Batman informs Gentleman Jim that in the future he becomes a ghost, angering the highwayman. Asteroth knocks Etrigan into the lava and he sinks out of sight.
Batman tries to warn Craddock that Asteroth will betray him, but Asteroth knocks him away. Gentleman Jim demands that Asteroth completes the ritual and the demon reluctantly agrees. However, Etrigan emerges from the lava and warns that demon deals are not to be trusted. Craddock tries to burn him with his iron cane, but Etrigan grabs the horn and knocks him away. Asteroth grabs Etrigan and then smashes him repeatedly. Batman distracts Asteroth with Batarangs but Asteroth unleashes gouts of unholy flame, knocking him back near Craddock's cane. Etrigan attacks once more and tells Batman to throw him Craddock's cane. Etrigan throws it into Asteroth's mouth, transforming Asteroth into dust. Etrigan then opens the horn, returning the souls back to where they came.
Blood prepares to send Batman back, but not before Batman informs Holmes that he's known as the world's greatest detective. Craddock is found guilty and hung, and he emerges from his grave, swearing vengeance on Batman no matter how long it takes.