Good girls go to heaven. Bad girls go to Sturgis.

That's what they say at the famous motorcycle rally. Is Mercy Watts a bad girl? Mercy doesn't think so, but she's going to Sturgis and things are about to go bad in a big way.

Mercy's grandad, Ace Watts, has decided to tick off one of his bucket list dreams, attending the famous Sturgis Motorcycle rally in South Dakota. It's also a chance to have a reunion with his buddies from Vietnam and to bond with his son. Predictably, Tommy gets a call from the FBI and can't go. To save her mother from wearing jeans and hanging with geezers, Mercy takes one for the team while escaping some problems of her own.

But new problems await her at the rally. After some odd interactions at Sturgis bar, one of Grandad's friends turns up dead and Mercy is expected to investigate. Despite her best efforts, she's drawn into her grandad's war with a string of deaths and a secret that the vets are intent on keeping no matter what.

Mercy's about to find out that a lie spoken fifty years ago can be used by a present day evil, and it's not only stalking Sturgis but the Watts clan itself.