A photographer creeps up on a butterfly in bare, wintry woods. The title gives its British name, Camberwell Beauty, while the poem uses the American one, mourning cloak. Both names suit a dark insect with pale edges on its wings.
The best image is the comparison of the open wings to a "two-page spread of darkness." The butterfly becomes a book, and the speaker a "book thief," eager and a little guilty, wanting to take something that isn't theirs. The camera is the tool for stealing it.
Then the poem shifts. Speaker and butterfly are alike: "quick to fly but loathe to leave." Both hover close to the thing they love and can't quite pull away. The last line, "And edged in light," could describe the wings, but it also seems to touch the speaker. The mood is quiet, patient, and full of wonder, with a small shadow of mourning that fits the butterfly's name.