Captain Joe Martin arrives in Aden with his wife Honor to replace the outgoing and much-loved Captain Nick Page of the Royal Military Police.

The NLF demand a swap. Alison watches over Ed, conflicted about their marriage. Honor confronts Martha. Yusra gets a coded message to Armstrong. Joe disobeys orders.

Joe's future in the army hangs in the balance. Armstrong fears for Yusra's safety. Honor is devastated by Joe's secret past. Markham struggles between duty and his own moral compass.