Menou's Warning Letter is the 8th episode of the D.N.Angel anime.
After a trip to the museum, Daisuke's grandfather sends out a warning notice that Dark will steal the Agate Links necklace. Takeshi Saehara goes to the museum to take a photo of it, but ends up photographing a mysterious girl he falls in love with.
As promised, Dark goes to seal the necklace, but meets the girl who begs him for one more day. Dark agrees and that night, Daisuke's grandfather, Daiki Niwa, tells Daisuke about a girl he met when he was Dark, 40 years ago, Menou Kurashina. Daiki gave the Agate Link to her the day before she died and her spirit has been sealed in it, waiting for him for eternity.
When Dark goes to steal the necklace the next day, Takeshi tries to stop him and Menou is touched by his kindness.
Dark steals the necklace, but turns back into Daisuke, that Menou mistakes for the phantom thief from 40 years ago she had been waiting for.