She had been thinking about moving abroad for ages, and last month she finally took the plunge.

took the plunge 是固定习语 take the plunge 的过去式形式，take the plunge 字面意思是"跳入水中"，比喻"下定决心做某事（尤指重大或困难的决定）"——经过长期考虑后终于采取行动。