take the plunge

To take the plunge means to do something important or very difficult that you have been thinking about doing for quite a long time. If you are taking the plunge, you are aware beforehand of the possible consequences or ramifications as it has been on your mind for a long time, and you are finally able to do it. This is a common phrase for individuals who have decided to get married, knowing it might be difficult but it is important for them to do it.