Contains spoilers for this episode and earlier episodes.
Lightman leaves the office Christmas preparations to join a mission in Afghanistan. Two Marines have been captured, and an American prisoner calling himself Khaled claims to have information about them. The soldiers suspect he joined the Taliban, but his shifting account suggests a concealed history. Lightman questions him while officials in Washington try to establish whether the people he names and the family story he offers are genuine.
The prisoner eventually identifies himself as Specialist Glen Welsh. Recruited from military prison for a covert assignment after the 2001 attacks, he was sent into Taliban territory and then abandoned when his tracking device stopped working. Lightman extracts the device to corroborate his story. Welsh also admits that he fought American troops after changing sides. Lightman recognises the betrayal without treating it as an excuse for everything the man did.
When the position comes under attack, Lightman refuses to leave Welsh behind. The prisoner takes up a weapon and helps cover the escape, dying in the fight. The Marines survive and thank Lightman for helping bring them home. Back in America, Cal challenges the officials who told Welsh's family that he died in prison and insists they hear the truth about his final actions. Emily welcomes her father back to the Christmas party, shaken by how close he came to dying.