A character or an entire group gets split. It can be a small rebellion of characters thinking differently or a Split Personality of a character creating his Evil Twin. The two entities created are so completely opposite that it's hard to believe they have something in common. Then, however, they reunite. The reunion results in creating something more perfect than the original entity. The main plot is often put in the time when the entities split. The reunion would be connected with the climax or resolution of the plot. The moment of the split would be often revealed in retrospection. Revealing this fact would be a plot twist.