step in
phrasal verb with step verb [ I + adv/prep ] uk/step/ us/step/ -pp-
插手；介入；居中斡旋
to become involved in a difficult situation or argument in order to help find a solution
[ + to infinitive ] An outside buyer has stepped in to save the company from going out of business. 一个外来买主介入，使公司免于破产。
When the leading actress broke her leg, Isobel stepped in and took over. 女主角摔断了腿，伊索贝尔临时加入顶替了她。