The Girl with the Broken Wing

The twins are in bed when it happens: they hear a sound like a wet
cabbage hitting the wall, a clattering on the roof tiles, then
a tapping at the window. It's a girl with a broken wing - and she's
come to stay.

Who is she? Where has she come from? And more importantly, what does
she want? The twins have no idea. Perhaps she's their guardian
angel ... but would an angel snore and have filthy feet?