The show works basically like this: there are 4 teams, who each have 17 minutes to put on a show. There are various musical numbers and a bit of plotline to string them all together. Kind of like a non-ABBA Mamma Mia.
This year's theme is "Flashback in Time", so each team was given a decade (50's 60's 70's and 80's) and their show is based around that theme. What team you're on is (mostly) determined by what department you're in.
I've actually never seen Flashback myself, but based on what I see in our show, it's cutesy, a bit cheesy (that'd be the Disney definition of a "bit" cheesy
), and humorous. Definitely musical. Plus we have Elvis.
By the way, Sadako is also in the show, and she's freaking hilarious. She has one of the main parts.