Abby Mills returns to Harper's Island for the wedding celebration of her best friend, Henry Dunn, and his fiancée, Trish Wellington. Abby hasn't been home in seven years, since her mother and five others were murdered by John Wakefield.

As the wedding party embarks on a scavenger hunt, Trish confronts an old flame, Henry makes peace with some locals and Abby runs into a friend who is haunted by John Wakefield and the island's tragic past.

When a local girl's death is ruled a suicide, Abby challenges her father, the Sheriff, to investigate what really happened. As the wedding approaches, Trish must come to a decision about Hunter.

The groomsmen take Henry fishing for his bachelor party and make a gruesome discovery. The bridesmaids celebrate Trish's final days as a bachelorette with a psychic who has a very strong reaction to Abby.

A father/daughter bike ride in the woods leads to a startling revelation. Abby uncovers the Sheriff's secret obsession as Trish and Henry's wedding rehearsal takes an unexpected turn.

With the wedding party reeling after the deadly events at the rehearsal, Abby, Henry and J.D. take matters into their own hands as they try and solve the murder.

As the wedding guests prepare to leave the island, evidence leads the Sheriff to believe one of them is responsible for the deaths on the island. Also, Abby learns a dark secret linking her to Wakefield.

A guest disappears as the wedding party prepares to board the ship for home. A mysterious phone call informs Abby that if anyone leaves the island, the missing person will be killed.

The remaining members of the wedding party begin to turn on one another. Then, after finding maps of the island's underground tunnel system, they all join together to search for a Madison.

While attempting to get to a boat that will take everyone off the island, a massive explosion destroys the marina. The group retreats to a hideaway where a secret is revealed that leaves the group reeling.

After being rocked by more deaths and the disappearance of another friend, the group fights back and is more determined than ever to track down the island's killer.

Eight out of the original 25 suspects strive to be one of the few survivors as all answers are revealed.

The tragic plot's final twist traps Abby as the true extent of the killer's cunning is revealed.