Antonio and Estrella are diving siblings. He works in maritime works, she studies underwater life and assists him. Its precarious economy could improve after finding cocaine hidden in the hull of a ship anchored in Huelva.

Four Spanish ex-intelligence officers are assassinated simultaneously worldwide. The CNI discovers all participated in the covert "Operation Ciénaga" decades ago in Colombia. Zeta, the CNI's best operative, must hunt and protect the sole survivor, facing his deadliest global mission yet. Top Colombian agent Alfa will join Zeta, seemingly knowing more about "Ciénaga"'s secrets than him.