Tom's skills as leader are tested to their extremes when they take part in an Extreme Emergency Response Initiative Exercise (EERIE) to test their response to nuclear, chemical or biological act in central London, but when the power in the grid is cut and Harry starts to feel ill, they soon begin to suspect it's for real.

The president of the USA makes an unexpected stop in London. Tom and his team are forced to work with the CIA. A power struggle between the two secret organizations begins, but will they stop fighting before a terrible mistake is made? And will Tom ex-girlfriend leave well alone or will she have to be taught a lesson?

Saturday morning. Danny and Colin practice their firing technique on MI5's gun range. Zoe and her fiancé Will are still madly in love. Ruth watches old movies at home whilst longing for real-life romance. A North Sea mission places additional stain on all relationships...

Zoe is tried for manslaughter when an MI5 operation results in the murder of an undercover police officer.