A reinvigorated Pete returns to Iraq with his girlfriend, Alex, at his side. In Syria, an unexpected snafu leaves PJ weighing his options. Facing inevitable defeat in both countries, ISIS is determined to cause as much damage as possible on its way down. However, signs of life are reappearing in areas freed from terrorist rule, causing volunteers to be hopeful.
PJ returns to Syria, inspired by Levi's sacrifice and intent on reaching ISIS' "capital," Raqqa. In Iraq, ISIS is losing ground in Mosul but stages a diversionary attack in the city of Kirkuk, where Italian volunteer Alex leads his men into battle against them. Meanwhile, Pete reaches a breaking point as casualties stack up in Mosul.
The long-anticipated siege of Mosul finally begins on the city's eastern limits. ISIS intensifies their tactics, meaning mounting casualties for the medics and an increasing emotional toll on Pete. In the U.S., PJ and Levi's family struggle to reckon with Levi's death, even as his body remains in Syria. ISIS is on the defensive in the Middle East, but progress comes at a steep price for the volunteers.
PJ's unit in Syria attacks the ISIS-held city of Al-Shaddadi, but heavy combat evades them once again; a frustrated PJ heads home to the States, leaving Levi and the others behind; Jeremiah seeks to do the opposite, as he misses war and is desperate to return; and all the volunteers do their best while joining Pete's medical team in preparation for the impending siege of Mosul in Iraq, but it soon proves fatal for one.
Itching for a fight, PJ transfers to an offensive unit in Syria, where he's assigned to liberate the strategic ISIS-held town of Al-Shaddadi. Meanwhile, in Iraq, medics Pete and Derek get dangerously close to the frontlines during a battle to push ISIS from a town outside their urban stronghold of Mosul.