Spain, 1950s. Montse's agoraphobia keeps her locked in a sinister apartment in Madrid and her only link to reality is the little sister she lost her youth raising. But one day, a reckless young neighbor, Carlos, falls down the stairwell and drags himself to their door. Someone has entered the shrew's nest... perhaps he'll never leave.

The R of the title stands for the young protagonist, Rune, fearlessly played by Pilou Asbæk. Imprisoned for violent assault, he's a cocky, good-looking young man placed in the hardcore ward, where his survival depends on quickly learning the prison's parallel world of rules, honor, and obligations. R also stands for Rachid, a young Muslim prisoner who becomes Rune's friend and accomplice, defying the rigid racial stratifications among the inmates.

A woman begins to suspect that her therapist neighbor's offer to free treatment is not the altruistic gesture it was meant to be.