The members of covert antiterrorism unit Section 20 are reunited by commanding officer Coltrane under the pretence of rescuing Dr. Helen McCluskey, a biochemist who's been taken as a hostage in Kosovo by the Albanian mafia.

After catching wind of a feud between Edon and his son Loric, S20 launches a raid on their business in hopes of extracting the location of the bioweapon. Loric's self-serving cooperation leads them to an exchange between Edon and jihadist brothers Zayef and Mahir.

S20 finds itself in Tel Aviv, where Kogan has escaped with a deadly piece of military tech that he plans to sell to Zayef and Mahir. The device, however, winds up in the hands of a notorious Israeli gangster who promises to make matters even more complicated.

Novin is forced to cooperate with Topal and Danny. After an explosive stand-off with Zayef and Mahir, Spiegel threatens to blow the lid off S20's operations in Tel Aviv, and later, the squad's arrangement with the local police reaches its tipping point.

S20 raids Zayef's hideout, leading to a confrontation that finds Mac defying orders to keep his team safe. Suspecting that Zayef plans to deploy the stolen military tech during a UN conference in Munich, S20 heads west, where a showdown of epic proportions awaits.

Long after the incident in Munich, Wyatt and Novin reunite with Mac who has since left the military; but while he lives out a seemingly idyllic existence with his wife and daughter, his violent past - and fears that his service was inadequate - haunt him.

Spencer joins S20 to take down Zayef for good, but when intel leads them back to the Demachi's, S20 finds itself embroiled in a feud that may expedite Zayef's plans and place military-grade weapons in the hands of terrorists across Europe.

S20 plans to storm a freighter carrying weapons, but when Spencer administers a kill order, Coltrane grows suspicious. As members of S20 contemplate following their conscience, Zayef divulges intel that could drastically change their fate.

With Chetri in the wind and Carolyn on their tail, Novin, Wyatt and Coltrane are forced to weigh loyalty against duty and self-preservation; still blacklisted and left with scant resources, S20 seeks the help of Russian allies to combat Russian foes.

After receiving shocking intel from an unlikely Russian informant, S20 decides to reunite for one final mission to track down Arianna and avenge their comrades. This time, however, they're doing things a little differently.