^ You mean - Umezu gets to know that Madoka cheated on him and starts to flirt with Mihara to maka a revenge?
.... well I was thinking more like this....
Mihara sees how faithful Umezu (I keep getting their names wrong >.<) to Madoka is. She starts feeling bad on how Madoka treats her relationship with him then sUmezu and Mihara had some accidental meetings. Mihara learned what true love is from Umezu. From then on she plotted a sinister plot... a plot involving Madoka, Umezu and a horrible accident that is about to happen.
... something like that :P