When high school teacher Tomás finds the bloodied, dying mayoress of his hometown high up on his local cliffs, he not only becomes obsessed with the case but turns into its first suspect. Though solving the case is local cop Maite's job, Tomás doesn't care if his own private investigation gets in her way. Solving the case might just give Tomás' life the purpose he has been silently dreaming of for so long. Though clashing more than once, the two of them come close as can be to finding the murderer: Norma, Tomas' very own physiotherapist. She killed the mayoress in a fit of rage after finding out one of her many dark secrets - one that lies many years behind but one that has deeply affected her and her family. A twist of fate has it, however, that Norma remains undiscovered; her former fear and powerlessness turn to empowerment and confidence. Then a second murder stirs up the waters. Now an unlikely team, Maite and Tomás resume the hunt for a murderer who is closer than they imagine.

Hierro is a crime drama mystery thriller that takes place on a remote island. The story revolves around the search for justice, the contrast between the inhabitants of the island, a murder investigation, and the suspects. As the truth is revealed, the small-community atmosphere is filled with distrust and suspicion, testing the neighbor-neighbor relationships and challenging traditional values.