Panel 1: The key is flat on the ground being dragged by the red string to the left.
Panel 2: A closed flat box with a keyhole in the front has a taut red string emerging from it.
Panel 3: The key is attached to the same red string and approaches the box as it's being pulled.
Panel 4: The key fits into the lock as the red string disappears inside the box.
Bottom right of the page: click