The antonyms for "on one plane" are "off-balance," "uneven," and "unsteady." These words describe a state of being that is not level or stable, and in contrast to "on one plane," they connote a sense of discomfort or instability. "Off-balance" suggests a lack of equilibrium, as when standing on one foot, while "uneven" implies a surface that is bumpy, irregular or inconsistent. Lastly, "unsteady" evokes the idea of something that wobbles or undergoes sudden shifts, such as a rickety bridge or swaying boat. Together, these antonyms help to emphasize the importance of balance and stability in our lives.