The series, based in part on real events and characters, takes us back to 1963 when a young man, Txabi Etxebarrieta, just 19 years old, is a poet and student of Economic Sciences at university who lives with his mother and siblings in the old section of Bilbao.
His older brother, José Antonio, 23 years old, had already been arrested for handing out leaflets five years before and collaborated with the organization ETA, founded in 1958.
When José Antonio falls gravely ill and is diagnosed with an inflammation of the spinal cord that keeps him virtually paralyzed, Txabi gets more and more involved in the organization's activities, collaborating with a group of young militants on sabotages and small explosions while he carries on with his specialized studies in Computer Science and a relationship with Julia, the younger sister of his brother's girlfriend.
In parallel, the Chief Inspector of the Political-Social Brigade (secret police) in Guipúzcoa, Melitón Manzanas, a resident of Irún and known for his torture of opponents to the Franco regime, begins to investigate this group of young activists.
Three years later, between 1966 and 1967, there is a strike in a cold rolling factory, in which 800 workers in Vizcaya feature in the dictatorship's longest labor conflict and which the Government ended by decreeing a state of emergency.
During this strike the 5th Assembly was held where Txabi was elected as one of the leaders of ETA and he decides to break away, leaving the University and his family life.
Once Txabi is elected as one of the leaders of ETA, this organization increases its activity, successfully carrying out bank robberies to finance their actions, becoming one of the targets of the Francoist police of the period.
On June 7, 1968, Txabi Etxebarrieta, accompanied by another militant in the organization, Iñaki Sarasketa, was driving in a Seat 850 on a road near Aduna when they were stopped by Guardia Civil traffic officer José Antonio Pardines, 25 years old.
Txabi murdered this Guardia Civil officer, who became the organization's first victim. Txabi and ETA had crossed "the invisible line".