Taylor Swift's giving her fiancé, Travis Kelce, a pretty impressive birthday gift ... she's putting out a whole music video all about the song everyone thinks is about their romance!
The singer-songwriter just dropped the vid for "The Fate of Ophelia" -- a song in which she likens herself to the character from "Hamlet" ... and, she credits Kelce for saving her.
The video starts in a hotel ballroom ... with a mural of Taylor as the doomed Shakespeare character on the wall -- before taking fans through a series of smoky jazz clubs and movie sets.