Shangri-La (S13E02)

Valerie Keenan, a young high school teacher, is found stabbed to death in the janitor's closet. The initial check of the surveillance tape doesn't turn up anything, but some of the students allude to the fact that another teacher, Mr. Bergen (Rob Campbell), was in a relationship with her. Bergen denies that anything was going on with Valerie and claims he was at the Mets game the night of the murder. The only problem is, the Mets game was rained out that night, but a local bartender verifies he stopped in. Then it hits Briscoe (Jerry Orbach) and Green (Jesse L. Martin); if it was raining that day, why weren't there any signs of rain (i.e. umbrellas, raincoats) on the surveillance tape? (iTunes)