Hamsters.
Gotta be hamsters.
To save the future, they go back in time to get hamsters.
Why?
Because they couldn't perfect this and had to revert to the original warp drive engine concept, for which it was easier to scoop up the waste material with.
Okay, with that aside, I'd forget the giant cylinder of doooooooooooooooooo*-- and get on with the Klingon undercurrent that had been building up in the previous film. Indeed, TVH as shown builds on this, albeit a little and with the dorky "you pompous ass!" moment in the 23rd century that makes Kirk out to be a double-triple dipshit when he tells Spock how one has to swear every other word in ours... unless that's the intended idea to shove more comedy up the 23rd century segment before they went to the 20th, har-har... But, as you're wondering, how do I know it makes Kirk look bad? Well, I don't -- but someone said it and the underlying context behind that phrase would give points to either a Klingon representative or a Federation one, or someone else trying to rile things up but the space drain thing from "Day of the Dove" was nowhere to be found here and it'd be "small universe syndrome" doubledummassery if it were.
(the thrill begins at 2:48)
That phrase needs to be anything but a notification tone... but either which way it lends more credence to TUC...
But all that aside, had TVH, overly long sitcom and all, had not been well-received, then we'd never have TNG - a show that actually took itself seriously all while the TOS movies continued to yuk it up. Which is amazing, considering it was the ha-har-haha stuff that had people flocking to this in the first place.
* Yup, it's footnote time again: