ESUS is an experimental unit set up to try and forge closer links between MI5 and the police force. The unit is headed by MI5 agents, but the rest of the team have been seconded from regional police forces because of their specialist undercover expertise. ESUS brief is to investigate cases that go beyond the rimit of either the regional police forces or special branch after ESUS. Five new rookies from the police force are assigned a drugs case. They are to prove back up for Barbara Hall who has been undercover for six months and is believed to be acting as a mule for Miguel Moreno who is thought to be negotiating a huge shipment of cocaine into Britain. When Barbara Hall fails to make contact on her arrival at London's City Airport, the team become suspicious. Has she gone OTS and joined forces with Moreno or has she fallen for his associate, the handsome Luis Rosselli?