This is easy: High dose benzos mixed with alcohol. Alcohol is optional, but adds to the second idea (plan made while drug taking hold, yet still fully functional cognitively)
Xanax or Klonopin are fairly well known for causing Anterograde Amnesia (look that up if you want a list of other causative agents). This requires high doses, but again... people have a strong tendancy to redose while on both of them, adding fuel to the fire.
They are also very easy to obtain in most drug-addled areas, both rural and urban.
This would follow suit of the latter idea-- At first, while the drug is taking hold, inhibitions are severely lowered, euphoria is felt, and users are quite talkative. Within an hour or two (Maybe more if klonopin was the agent), the benzos would take hold, eliminating any sense of reason and causing the users to enter a state of technically functional yet dangerously uninhibited stupor. I'll tell you a story of my own quickly (a real one):
I was babysitting a kid when I was 19. I was a pretty responsible kid, and didn't really use drugs.... but I got bored and curious. I started looking through the lady's medicine cabinets after she told me I could take an ambien to sleep if I wanted to. While looking around, I discovered klonopin for the first time. I took a few, knowing what it was and how ativan helped me when I had panic attacks in the past... I took it to relax.
Well, I was sitting on my laptop... and I was reading through some online articles and talking to friends... and I nodded off while sitting. In what felt like a few seconds, I woke back up because my back hurt. I thought "Man, that made me tired fast, geez."
I went back to bed... the lady came home, and I went home as usual, and decided to go back to sleep again (She came home at around 5am). I then noticed something strange as I wsa going to bed..... I wasn't wearing any underwear.
"Well..... that's.... weird.................." .... Cargo shorts with no underwear? That's just.... something that doesn't happen. I couldn't fathom how on earth I'd forgotten to wear my boxers. I couldn't wrap my head around it. Suddenly, I got a text from the mom.
"Jim, what happened here last night?"
"... Uhhh.... what?"
"My pills are scattered all over the upstairs from the bathroom to the stairs." ---> .... What the....?
"... and I found a pair of white boxers in the bathtub."
O.O .... oh GOD! I KNEW I had underwear on! .... oh GOD what did I do?!
Turns out I didn't just fall asleep in place at the computer. I'd awoken as a zombie, and proceeded to start arbitrarily raiding the house looking for other drugs and items to take. I'd completely lost my shit, and started opening her pill bottles and dumping them everywhere, all over the floor, and throwing them down the hallway-- right outside her 5 year old's door. I have no idea what prompted me to remove my clothing, or what on earth I was thinking.... but I did not remember ANY of it until several months later while randomly in the shower, when suddenly the imagery came back to me in pieces. To this day, I still only remember pieces of that night.
Needless to say, I never babysat again. I could've killed the kid.
Lastly, just to add, I did have a good few conversations online after I originally thought I'd nodded off. All were perfectly coherent, but I'd forgotten all of them.
Hope it helps! =)