Here is a great overview of the story including the 'possible' historical motivation for the same,
Of course you can get the Wikipedia version of the story as well
And the barebones of the story are as follows
In the original version of the tale, Todd is a barber who dispatches his victims by pulling a lever as they sit in his barber chair. His victims fall backward down a revolving trap door into the basement of his shop, generally causing them to break their necks or skulls. In case they are alive, Todd goes to the basement and "polishes them off" (slitting their throats with his straight razor). In some adaptations, the murdering process is reversed, with Todd slitting his customers' throats before dispatching them into the basement through the revolving trap door. After Todd has robbed his dead victims of their goods, Mrs. Lovett, his partner in crime (in some later versions, his friend and/or lover), assists him in disposing of the bodies by baking their flesh into meat pies and selling them to the unsuspecting customers of her pie shop. Todd's barber shop is situated at 152 Fleet Street, London, next to St. Dunstan's church, and is connected to Mrs. Lovett's pie shop in nearby Bell Yard by means of an underground passage. In most versions of the story, he and Mrs. Lovett hire an unwitting orphan boy, Tobias Ragg, to serve the pies to customers.
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