“It was a mermaid who rose to greet her in her own full-length mirror,” reads Ian McEwan’s novel “Atonement,” of the moment in which Cecilia Tallis, in 1930s England, donned her “green backless post-finals gown” for dinner. In the 2007 movie, that dress became a reality — and, worn by Keira Knightley, one of the most beautiful movie costumes of recent decades.

We don’t see so many costume dramas in cinemas today — they have, alas, gone out of fashion — but it’s uncanny how a gown can stay with us, and this one, which has never left my memory, seemed right to revisit for the early days of this weekly column. Designed by Jacqueline Durran, the dress doesn’t strictly adhere to the styles of the ‘30s (the detail at the bodice, for example, is laser cut rather than beaded), but is instead an homage to an era; a slippery, almost watery gown that seems to float over its wearer, captured in a green that’s almost eerie in its brightness, as if you could see it in the dark. It seems to move of its own accord — you can almost hear the fabric whispering. This dress, you imagine, could tell a few secrets.

Moira Macdonald, Seattle Times arts writer