After completing 15 years of prison term, falsely prosecuted an innocent taxi driver begins his deadly game of vengeance with the mob boss he considers responsible for his family’s murder. But on the course he discovers his daughter is still alive and in their captive.

When does he discover that his daughter's still alive? If it's pretty early in the story then consider making this the inciting incident.

Richiev's version reduces the word count considerably and does a great job of summing your story up but is his goal to rescue his daughter or kill all the mobsters? If the inciting incident is finding out his daughter is still alive then the goal is to rescue her. If that's saved for a twist (Act II to III?) then consider not including the fact that his daughter is alive but simply allude to it.

If he's in prison for 15 years for murdering his wife and daughter isn't 15 years a pretty lenient sentence?

What's going to make this story stand out from other similar stories? Where's your hook? Think about Oldboy - the guy was imprisoned for 15 years and he has no idea who did it or for what reason so he sets out to simply find out the answers. It's so simple yet immediately gets you interested.

Hope this helps.

"Released from prison, an ex-con seeks revenge on the mob boss who framed him, but first must rescue his daughter who's held captive by the Mafioso's minions."