Frasier: (cautioning Roz about Bulldog's real intentions for wanting her to be his producer) Oh, Roz, Bulldog knows the blunt approach won't work with you, so he's being more subtle. But his ultimate goal remains to... well, to...
Roz: To what?
Niles: To play Aeneas to your Dido.
Dido and Aeneas, by Henry Purcell, is England's oldest opera, based on a story by Virgil. In it, Dido falls in love with Aeneas but he leaves her, returning to his duties as a soldier. Heartbroken, Dido commits suicide. In other words, Niles is saying in his very "Crane-like" way that Roz is nothing more than a one-night stand to Bulldog.