This was taken from the manga, but the story line is exactly the same.
Conan, Ran, and Sonoko attend their middle school teacher's wedding. Before the ceremony is to begin, the bride, Sayuri Matsumoto, is found lying on the floor poisoned. The poison from the warm can of lemon tea she was drinking. The suspects are rounded up and the evidence is collected. After looking at all the evidence, Conan relized the criminal is none other than the groom, Toshihiko Takasugi. Using Sonoko to reveal the criminal to Detective Megure, the true motive is discovered. The groom lost his mother 20 years ago in an accident when the criminal being chased by Sayuri's father, police superintendent Kiyonaga Matsumoto, hit Toshihiko's mother. Instead of helping his mother, the superintendent chased the criminal. As a result, his mother died. Until he met Sayuri at a college campus, Toshihiko lived his life normally. Once he discovered that Sayuri's father was the one who allowed his mother to die, the flame of revenge burned in him. Toshihiko wanted the superintendent to know how it felt like to lose someone important... Just then Kazumi slapped Toshihiko, angrily shouting that Toshihiko was the one who didn't know anything. Sayuri knew about the accident, but yet she wanted to marry Toshihiko. Toshihiko didn't understand even after watching her drink her warm lemon tea all those times. In Sayuri's childhood, she was bullied by some boys and the one who rescued her was a neighbor boy. And he used to secretly bring warm lemon tea from his parents store to her. And ever since then Sayuri drinks warm lemon tea to give her courage... Toshihiko was the one who protected Sayuri 20 years ago. Toshihiko was Sayuri's first love! And when Toshihiko poisoned the warm lemon tea, Sayuri knew that he did. She drank the poison willingly. Then the door opened and a police officer informed the group that Sayuri was going to be all right. Toshihiko cried tears of joy... Two months later Sayuri made a complete recovery and three years after that she married the one she thought of for all those 20 years...