Fire and hemlock - Diana Wynne Jones

In the mind of a lonely, imaginative girl, who can tell where fiction ends and reality begins? An epic fantasy, spanning nine years

The fire and hemlock photograph above Polly's bed sparks memories in her that don't seem to exist any more. Halloween; nine years ago; she gatecrashed a funeral party at the big house and met Thomas Lynn for the first time.

Despite the fact that he's an adult, they struck up an immediate friendship, and began making up stories together - stories in which Tom is a great hero, and Polly is his assistant. The trouble is, these scary adventures have a nasty habit of coming true

But what has happened in the years between?
Why has Tom been erased from Polly's mind, and from the rest of the world as well? Gradually Polly uncovers the awful truth and, at Halloween nine years on, realises that Tom's soul is forfeit to demonic powers unless she can save him.