The light of the moon is shining on a guard who's been on duty all night. The moon light makes him so warm and comfortable that he forgets where he is and starts to undress. He thinks he's home! Everyone is going to see him take off his clothes!
You can't seem to get the guard to stop undressing so you throw some darts (だつ) at him. You figure that even if the guard doesn't snap out of this weird sleepy trance, at least you can use the darts to stick his clothes to his body. Then maybe he won't be able to undress anymore.