S17·E5 Common Language

A young Russian girl leads Dale Smith out of a nightclub, where he is promptly attacked by three men. Despite Smith's protests, Supt. Chandler assigns DI Cullen to investigate the assault. They track down the Russian woman, Natasha Ivanova, through a language school which seems to be recruiting students to work as prostitutes. Natasha is desperate as her visa is about to expire, and her sister has disappeared. Her sister is found murdered, and Smith realises the men responsible for her death and his assault are linked to a man from his old army unit (episode: Blurred Around the Edges).