Lou Bertignac has an IQ of 160 & a good friend in class rebel Lucas. At home her father puts a brave face on things but cries in secret in the bathroom while her mother rarely speaks & hardly ever leaves the house. To escape this desolate world Lou goes often to Gare d Austerlitz to see the big emotions in the smiles & tears of arrival & departure. But there she also sees the homeless meets a girl called No only a few years older than herself & decides to make homelessness the topic of her class presentation. Bit by bit Lou & No become friends until the project over No disappears. Heartbroken Lou asks her parents the unaskable question & her parents say: Yes No can come to live with them. So Lou goes down into the underworld of Pariss street people to bring her friend up to the light of a home & family life she thinks.