The teacher unrolled the plot of the story step by step for the students to understand better.

unrolled 是动词 unroll 的过去式。unroll 由前缀 un-（表示"相反动作"）和动词 roll（卷起）组成，字面意思是"展开卷起的东西"。在这里比喻为"逐步展开、揭示"故事的情节。