When I was little (1970s plain chocolate and dark chocolate were the same thing.
Now a lot of people use plain to mean milk chocolate with no fillings/extras. So for example cadbury's dairy milk might be described as plain. kit kat = not plain as it has wafer in. Strawberry cream again not plain as it has a filling.
Ime if the person is 40 or older, they probably mean dark chocolate if they say plain. If you can't check with the person then buy wine or shower gel instead.