As implied from the headline, this is a revenge drama.
The story is about a teenage girl falsely implicated in the murders of her parents, released from prison after 10 years and going on revenge mode to avenge her wrong imprisonment and murder of her parents.
The plot: Taking a u-turn from usual revenge dramas, this drama focuses on the main leads getting revenge by conning the bad guys. As said above, FL is implicated in her parent's murder but found support in an able lawyer who has the problem of being overly emotional with the cases. FL is supported by a bunch of her "colleagues"and together they formed a perfect team to avenge for what they had to suffer in the past.
Acting: This is the best thing about this drama, the acting. Everyone has done a splendid job here. I loved the dynamics between Ro-Um and Moo-young.

This drama do suffers from the obvious low moments in the plot which can be overly complicated at times. But the biggest positive is that the writers were able to wrap things up perfectly. In an era where kdramas are sucking at the ending, this drama feels like a breath of fresh air about how they approached the story and how well the ending is written.

I feel like the ratings are not doing justice to this drama. We have mediocre products like movies of certain big stars getting ratings of more than 8, while such good dramas are stuck at 7.7.
Don't go with the ratings. You will surely enjoy this drama.