Episode 50 ran in the U.K on November 21 2017 and in the U.S. on February 23 2018:
Meanwhile, General Lázsló shuts down Section 20, forcing Donovan to work in secret. She discovers that Zaryn is in fact Karim Markov, a Russian scientist who allegedly killed his colleagues with Novichok, a nerve agent they invented.
Episodes 51 ran in the U.K on November 28 2017 and in the U.S. on March 2 2018:
Section 20 track Berisovich’s meth lab in Turov where Markov is making more Novichok and destroy it, though Berisovich escapes with Markov.