Karen, a maths teacher, finds herself involved in a traffic accident for which she is not to blame. The accident causes the death of a girl on a scooter. Although innocent, the young woman is distraught, more so when she discovers that the dead girl - Miriam - is none other than the sister of her pupil, Lukas. The more everyone around her assures her that she is not to blame for the girl's death, the guiltier Karen feels. To her, the accident is like an equation that just doesn't add up. In an attempt to make up for her actions at least in part, she begins to give Lukas extra maths lessons. But, consumed by his pain, the boy wants much more from her.