It's an ad for Planet Fitness, and the theme is that the gym has turned Lindsay's life around from train wreck to the picture of mental and physical health.

The voiceover is hilarious ... Lindsay has gone from DUI to DIY. She's so mentally acute she destroyed on "Jeopardy!" And the turnaround has left the paparazzi in tears!!!

There's a pretty amazing supporting cast in the 30-second ad -- William Shatner, Dennis Rodman, Buzzy Cohen and Danny Trejo ... all touting the virtues of exercise and fitness.

BTW ... NBC said Thursday all the Super Bowl ad slots have been filled, and it ran companies that advertised between $5.8 million and $7 million for 30 seconds of airtime.

Lindsay's been under the radar for a few years now, so the ad is actually more effective because of it. It's kind of like a reveal on "Extreme Makeover" ... except a lot more cerebral. It's actually a great P.R. move for her, because it's believable ... somewhat.