D&D: help, what is plot

Soo I was stupid and underestimated plotting a D&D campaign. The players keep moving parts around, you constantly have to adapt, and you're under pressure from session to session, which in my case means I only have a week to prepare and that's NOT a lot of time. I have no idea how our previous DM did this, and for such a long campaign. (At least I wrote weekly summaries. This time one player promised to write summaries but hasn't yet, which makes keeping track of things even harder.)

Because I underestimated it, I wasn't sufficiently prepared when the campaign started and only figured out later that the plot in its current form has huge holes. Now I have to adapt. I managed to buy some time with side quests, like an extra dungeon, at least that was a lot of fun. But now my players are in the middle of a detective plot (whyyyy did I think this was a good idea) and I really need to figure out who's behind it. Ahem. (My players: "it would be nice to get to some actual plot." Also my players: "nah, neither of these quests on the board looks cool, are you sure there aren't any others?" No there are not.)

Soo I would be grateful for feedback and brainstorming help :) Just writing it down will probably help.( Probably it will all end in chaos. )