What plug are you talking about? Is there a plug in the trap? Is that what you are referring to?

To release the pop-up stopper, you remove the actuator arm from the back of the drainpipe underneath the sink. Unscrew the retaining collar and remove the stem. You may need to first release the end of the stem from the vertical actuator.

Thanks, Randy. I have to admit that I don't know what all that means (I have no idea what an actuator might be - it sounds like a kind of life-coach), but the main thing is that you're telling me that it's not a simple matter of unhooking something, which is what I expected. I thought there was something wrong that I couldn't see anything that simple. Now that you told me that I'll need a screw driver, I'll take another look.

No screwdriver needed. Maybe pliers if it's really stuck. It's a threaded collar that you are removing.
The actuator is a small diameter rod that sticks out the back of the drain assembly. When the rod is pushed down, the stopper goes up. And vice versa.

The vertical rod usually attaches to a somewhat horizontal rod that goes into the drain with a nut holding it in place. with some stoppers, the end of the rod goes into a hole at the end of the stopper. Movement of that horizontal rod up and down causes the stopper to rise and fall. Sometimes to disconnect it you raise it to the highest then give it a little twist to free it from the end of the rod.