Everyone is a suspect in the murder of Charlie Stoddard, especially Frank and Elena. The Sheriff seems to have some emotional tie to Elena/Esmerelda, and the truth soon comes to light: she spent 7 years in prison for killing an ex-boyfriend who abused her during their relationship, and the Sheriff handles her case to the extent of providing her with a new identity.
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.
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.
Michael finds something unexpected on the tundra, and Dan is confronted with what he is becoming, just as a heartbreaking discovery changes the town forever.
As the town attempts to come to terms with the recent tragedy, Petra and Ingrid seek the truth. Meanwhile, Dr Khatri moves a step closer to achieving her goal.
Dan faces his toughest battle yet as someone makes the ultimate sacrifice for the greater good, while Michael and Ingrid venture out in search of justice.
Oslo send two new officers to attempt to make some sense of the anarchy in Fortitude, and Dan Anderssen continues his descent into madness, bringing him face to face with a grief-stricken Michael Lennox.
A survivor of the deadly parasite makes a connection with Michael, but things take a violent turn. Meanwhile, Dan continues to lose his grip on reality.