Lupin:
Sirius here is a friend of mine
I see, all these years, I've been blind
I just didn't understand (that)
They arrested the wrong man

Yeah, the traitor, and the murd'rer, too,
His name is Peter Pettigrew
Seems that he was telling lies,
Faked his own demise

Sirius:
That thing's not a rat!
That thing's not a rat!
He's a wizard, Harry—
That thing's not a rat!

That thing's not a rat!
That thing's not a rat!
I'm tryin' to tell ya, Harry—
That thing's not a rat

Lupin:
Listen, Harry, can't you see?
He betrayed James and Lily
We thought that they chose Sirius
When they did Fidelius

Sirius:
And the only reason that they're dead
Is because they picked Pete instead,
Seems he was the Dark Lord's spy
Gave them up to die

Both:
That thing's not a rat!
That thing's not a rat!
He's a wizard, Harry—
That, that, that, that—

That thing's not a rat!
That thing's not a rat!
He's an Animagus,
That thing's not a rat!

Lupin:
Did you ever wonder how
Ron's plain old house rat,
Scabbers, could live so long?

Magic kept him alive
These twelve long years, now,
Hiding cause his plan went wrong...

'Member, Harry, when you saw
A finger missing from his paw?
Peter cut it from his hand
Killed the Muggles, then he ran

Sirius:
And you know that I just had to laugh
Cause they thought I did it myself
Like I wouldn't rather die
Than be Voldemort's spy

That thing's not a rat!
That thing's not a rat!
He's a wizard, Harry—
That, that, that, that—

That thing's not a rat!
That thing's not a rat!
You gotta believe me, Harry,
That thing's not a rat!
Yeah!

Both:
That thing's not a rat!
That thing's not a rat!
Harry, Harry, Harry—
No, no, no, no,

That thing's not a rat!
That thing's not a rat!
Let us show you, Harry—
Oh, oh, oh...