As drug deaths surge in '90s Britain, the government turns to a veteran customs investigator to recruit a novice unit for an unprecedented covert mission.

Customs recruit Guy faces tests that threaten to blow his cover. Meanwhile, the team in Liverpool chase the scent of a suspicious bakery.

A volatile alliance puts Guy at the heart of a high-risk smuggling deal. The Legends in Liverpool embark on a surveillance mission that ends in trouble.

Cracks start to appear in both gangs as tensions, pressure and paranoia run high. Kate looks to exploit a possible weakness in Carter's armour.

While Kate chases a murky trail of corruption, a potential meeting with a feared drug lord sends Guy, Bailey and Don on a dangerous overseas mission.

Betrayal, distrust and ambition put the entire mission — and the Legends — at risk. Can they stop an unprecedented heroin shipment from reaching Britain?