How does this relate to Dark Shadows tv series?

Quentin and Angelique were in the original tv show. Grayson Hall played Dr. Julia Hoffman in the show but had the role of Carlotta in the movie.

The basic plot of the tv show was Willie Loomis releasing Barnabas Collins from his coffin. I believe the show had already been on for a while before this took place. Barnabas visits Collinswood as a cousin from England, a direct descendant of the original Barnabas. Here the plot splits away from the tv show. The tv Barnabas didn't kill and only took enough blood from his victims to survive. The movie Barnabas didn't hesitate to kill.

Think of the movie as separate from the TV series.

As the other person said, Quentin and Angelique were in the original series but not Carlotta. One storyline of the original series is called "Parallel Time" where there is a room people can go to and look at alternate versions of themselves; who and what they would be if they had made different choices in life. I like to think of the movies as just another band of parallel time myself, but they stand alone and have very little connection to the show other than using the same characters and actors.