We have been to this restaurant many times when visiting Bozeman, Montana. At peak times, especially during the weekend between approximately 6 and 8 p.m. I would recommend a reservation. Otherwise, there are usually tables available and, at the very least, there is almost always room at the bar.
I spent a week in Bozeman three four months ago and dined at Plonks four times. I will say that I always found a seat at the bar though it was crowded each time. At the time I would arrive there the room was near full and a reservation was good to have. It is a very large room so eventually something will open up. That is my experience. Great place to eat and drink.
I have eaten there many times and never had a reservation nor have I been asked if I had a reservation. If you are going for dinner and you arrive before 7 you should be fine.
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