In the remote Arctic town of Fortitude, two children make a remarkable find, while the discovery of a horrific crime brings a new arrival to the island that complicates Sheriff Dan Anderssen’s investigation.

Sheriff Anderssen and the police close in on their murder suspect, while DCI Morton follows a different line of enquiry.

Dan and Frank go in search of a suspect; however, Dan's feelings for Elena are threatening to overwhelm him. Morton, meanwhile, uncovers a new piece of evidence.

Following Dan's loss of control, Morton takes over the murder investigation. Elsewhere, Elena's mysterious past is about to catch up with her.

Shock waves reverberate around Fortitude as the killer is revealed, while Morton tunes his attention to the death of a scientist on the island months earlier.

While Morton uncovers new information about the death of Billy Pettigrew, Hildur makes a heart-breaking discovery. Another gruesome event occurs in Fortitude.

Panic sweeps through Fortitude as news of the second attack spreads. Dan and Morton must work together while Frank hopes the latest events will exonerate Liam.

While Frank takes the law into his own hands, DCI Morton closes in on the truth about Pettigrew's death. Vincent and Natalie also make a discovery.

DCI Morton pursues Henry Tyson into the Arctic wilderness in search of the truth, while worrying test results force Governor Odegard to take drastic action.

In the penultimate episode, the truth about Billy Pettigrew is revealed. In the lab, Natalie and Vincent probe the cause of the disturbing events in Fortitude.