CBS' new murder mystery "Harper's Island" has a large cast ... and it needs it, because each week for the 13-week run of the show at least one person gets murdered!

The mystery begins when Henry Dunn (Christopher Gorham), and his fiancée, Trish Wellington (Katie Cassidy), take a group of friends and family to Harper's Island for their wedding celebration.

"Henry feels this is going to be the best day of his life," Christopher says. But he soon learns otherwise.

The island has had a murder spree in its past, leaving six dead. And while the murderer was caught, the trauma of it sent Abby (Elaine Cassidy), who lost her mother, off the island until the story opens. Abby is Henry's best friend, so she bites the bullet and returns for the first time for the wedding.

"As soon as we find out one person is dead, we are going to be running and screaming and freaking out all over the place," says Katie, whose real-life dad is David Cassidy.

"Harper's Island" premieres tonight on CBS at 10 p.m., but the killer won't be revealed until the July 2 finale.

"They won't tell us who is going to die, or when they are going to die," Christopher adds. "They won't tell us who the killer is. It could be anybody!"

Check out TV.com for Harper's Island behind-the-scenes videos, episodes, exciting photos, & more!