Abby Mills returns to Harper's Island for the wedding celebration of her best friend, Henry Dunn, and his fiance, Trish Wellington. Abby hasn't been home in seven years, since her mother and five others were murdered by John Wakefield.
The wedding party heads out on a scavenger hunt. While out, Trish meets up with an old flame, Henry. Abby talks to the locals and runs into a friend who has been haunted by John Wakefield.
When a local’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 an unexpected situation. Abby uncovers the Sheriff's secret as Trish and Henry's wedding rehearsal takes an unexpected turn.
With the wedding party reeling after the events at the rehearsal, Abby, Henry and J.D. decide to take the investigation into their own hands.
The wedding guests begin preparing to leave the island but the Sheriff suspects that one of them is the killer. Some new information reveals an eerie connection between Abby and Wakefield.
As the guests get ready to leave the island, one guest mysteriously disappears. Abby receives a phone call from someone threatening to kill the missing guest if anyone leaves the island.
Still stuck inside the Candlewick Inn, everyone starts to turn against each other. However, they find a map of the tunnels underneath the hotel and are forced to come together to find the missing guest.
An attempt to leave the island turns into a disaster. The group makes a horrifying discovery.
The group narrowly escapes the Cannery and reunites at the church thinking it's safe — until someone else goes missing.
As the killer's murderous rampage continues, the group starts fighting back when they discover a possible way off the island.
In the finale, the remaining members of the group battle for survival as the truth is finally revealed.