Led by Major Harry Markham, the Royal Military Police assist special forces on a dangerous mission. Mary Markham experiences complications with her baby's birth, and Alison Laithwaite introduces Honor to the BP Club.

Captain Joe Martin meets with American journalist Martha Franklin to demand that she hand over incriminating evidence. Christmas celebrations at the Royal Military Police base are cut short by a shocking discovery.

The RMP begin their desperate search. Tilbrook takes Markham to visit an important prisoner. Alison can't go on with her pregnancy. Alison confides in Honor about her affair. Ed receives intelligence that leads him into danger.

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.