DI Carl Harrington is appointed the new leader of the elite specialist undercover squad and his first case is that of suspected drug smuggler Lance Anthony Izzard, who is shipping a large consignment of drugs across the channel in a tanker.

Detectives Jake Brown and Carl Harrington have been recruited to join the Elite Specialist Undercover Squad, an experimental unit of Britain’s best undercover police officers, headed by MI5 agents. But after an undercover officer fails to report in, the whole team falls under suspicion.

Suspicious of agent Hughes’s involvement with Barbara Hall, Brown begins his own investigation. But unbeknownst to Brown, Edna suspects that Moreno’s criminal enterprise reaches far beyond the drug trade.

When the junior education minister commits suicide, it's only the latest in a series of scandals involving government officials. ESUS is asked to investigate the private club that all the officials had belonged to.

After the Vice Squad’s raid, the ESUS team figure out the next steps for their operation, while Pauline receives a mysterious offer.

Agent Frankie Li goes undercover on the security staff of Brian Ha Sung, a Hong Kong businessman and smuggler. When Harrington and McEwan intercept one of Ha Sung's shipments, they find it carries highly unusual cargo.