Poor Thing is the third song in the movie. It is sung by Mrs. Lovett.
During this song, we learn the name that Sweeney Todd used to go by, Benjamin Barker, and we're treated to a flashback which has Lucy Barker, Benjamin's wife, being brought to the house of Judge Turpin by Beadle Bamford, who tells her that the Judge is remorseful for what he has done to her husband. Unfortunately for Lucy, there's a wild masked ball well underway when she enters the Judge's mansion. Confused and disoriented from the sights and sounds of the party and from the drinks that she has, Lucy winds up in the hands of Judge Turpin himself, who is anything but remorseful, and who has used this ball as a means to get her alone and defenseless. The Judge rapes Lucy amid the cruel laughter of the gathered crowd, none of them having any mercy upon her.
Sweeney, horrified at this despicable act, is further grieved to learn that Lucy poisoned herself soon after the rape, and that Judge Turpin kidnapped and adopted his daughter Johanna as his own. His family and former life all but destroyed, Sweeney vows vengeance against Judge Turpin and the Beadle.
In the stage musical, the song is a little different. It's mentioned that both the Judge and the Beadle wanted Lucy, and there's a verse just before the main action of the Beadle inviting Lucy to the Judge's house which mentions the transportation of Benjamin Barker in Mrs. Lovett's own words and is a little more indicative of how little Mrs. Lovett thinks of Lucy.
“You’ve got a room over the shop, haven't you? If times are so hard, why don't you rent it out?”
“There? No one will go near it. People think it's haunted.”
“Yeah. And who's to say they're wrong?
You see, years ago, something happened up there.
Something not very nice.”
There was a barber and his wife
And he was beautiful
A proper artist with a knife
But they transported him for life
And he was beautiful…
“Barker, his name was. Benjamin Barker.”
“What was his crime?”
He had this wife, you see
Pretty little thing
Silly little nit
Had her chance for the moon on a string
There was this Judge, you see
Wanted her like mad
Every day he’d send her a flower
But did she come down from her tower?
Sat up there and sobbed by the hour
Ah, but there was worse yet to come, poor thing
Well, Beadle calls on her, all polite
Poor thing, poor thing
The Judge, he tells her, is all contrite
He blames himself for her dreadful plight
She must come straight to his house tonight!
Poor thing, poor thing
Of course, when she goes there, poor thing, poor thing
They're havin' this ball all in masks
There's no one she knows there, poor dear, poor thing
She wanders, tormented and drinks, poor thing
The Judge has repented, she thinks, poor thing
“Oh, where is Judge Turpin?” she asks
He was there, all right --
Only not so contrite!
She wasn't no match for such craft, you see
And everyone thought it so droll
They figured she had to be daft, you see
So all of ’em stood there and laughed, you see
Would no one have mercy on her?”
“So it is you. Benjamin Barker.”
"Where is Lucy? Where is my wife?"
"She poisoned herself. Arsenic, from the apothecary down the corner. I tried to stop her, but she wouldn't listen to me. And he's got your daughter."
"He? Judge Turpin?"
"Adopted her. Like his own."
"Fifteen years, I've sweated in a living hell on a false charge.
Fifteen years dreaming that I might come home to a wife and child."
"Well, I can't say the years have been particularly kind to you, Mr. Barker -- !"
"No! Not Barker. That man is dead.
It's Todd now - Sweeney Todd.
And he will have his revenge.”