The four Casson children, whose mother, Eve, is a fine-arts painter, have all been given the names of paint colors. Cadmium (Caddy), is the eldest; then Saffron (Saffy); Indigo, the only boy; and Rose, the youngest. When Saffy discovers quite by accident that she has been adopted, she is deeply upset, though the others assure her that it makes no difference at all. Saffy is the daughter of Eve's twin sister, who lived in Siena, Italy, and died in a car crash. Grandad brough Saffy, as a very small child, back from Siena.
At Grandad's death he leaves something to each of the children. To Saffy, it is "her angel," although no one knows its identity. How Saffy discovers what her angel is, with the help of an energetic new friend, lies at the heart of this enchanting story.