With the information from Ana that her husband Abdul Kharami is onto the CIA's plan to provide Iran with flawed schematics for a nuclear trigger, MI-5 also learn the Iranians have acquired completed circuit boards that will allow them to proceed with their plan. They know the courier with the boards will be traveling to Iran on a British airline but the Iranians have put in place a fail-safe - any attempt to stop the courier will result in the poisoning of London's water supply. Ros and Adam board the aircraft while Jo and Malcolm try to locate the poison. Meanwhile, there is a concern that Section D may have a mole.
—garykmcd