A few years ago, I was seeing someone who was watching NCIS, and this came up as the episode description for the next one:

For those who don’t know, the A-plot is the most important plot in a story; the B-plot is the next-most-important; and the C-plot, which is optional, is the least important. They’re all supposed to tie together somehow, or at least be somewhat relevant to each other — like how in the “Dear Doctor” episode of Star Trek: Enterprise the A-plot (which takes a while to get going) is Dr. Phlox’s ethical dilemma, where the B-plot is his potential relationship with Crewman Cutler. In the episode, Phlox refers to the different morality of his species vs. human morality, and then he has to undergo his own moral dilemma.

But I can’t see how those two plots in the image above could possibly relate to each other. It feels more like the juice cleanse subplot was added for humor’s sake, to lighten the serious mood of the A-plot, which is the investigation into the murdered officer. It’s been a long time since I’ve watched Star Trek: Voyager, but I seem to remember a lot of episodes with a B-plot that was almost totally unrelated to the A-plot. Just throwing in a B-plot to lighten things up is a waste of space; sure, the viewer (or reader) might enjoy it, but the best B-plots and C-plots are in service to the A-plot in one way or another.

Keep this in mind when you’re writing your stories.