Philip Esdaile has a secret.

Beneath his Chelsea house, in the cellar below his studio, a tiny forgotten hole in the floorboards has created a camera obscura. When the lights are off, the room becomes a dark chamber, projecting an inverted image of the glass roof, the sycamore, and the London sky onto the concrete floor below.

For three years Philip has kept it to himself.

Then, on the morning of his farewell breakfast before a three-month sabbatical, he goes down to the cellar for a bottle of orange curaçao and takes one last look at the image only he has ever seen.

Two shadows cross the projection.

A parachute crashes onto the glass roof above.

One fallen airman moves. Raises an arm. There is a flash.

The other man never moves again.

Upstairs, Philip’s studio becomes the scene of an impossible accident: two test pilots tangled in parachute silk on the roof, one dead, one barely alive. The police see an aviation incident. The news calls it engine failure. But Philip has seen something else — a murder witnessed only as light and shadow on a cellar floor.

Among the guests trapped inside the house are a naval commander with secrets of his own, a barrister who knows how quickly friendship becomes liability, a journalist who recognises the shape of a buried story, a frightened photographer who removes evidence from the roof, and a young woman whose injured lover may be the only surviving witness.

At the centre of it all is Captain Owen Maxwell, the dead man on the glass: a test observer who had spent fourteen months gathering evidence of illegal arms sales, falsified authorisations, and a defence scandal reaching into the highest levels of government.

The obscura shows Philip the killing.

But proving what the shadows meant may destroy everyone who saw them.