About Birdsong Season 1, Episode 1 :

Stephen and Isabelle start an illicit affair, with ends up having huge consequences for them both. Years later in 1916, Stephen finds himself serving on the Western Front in the very place where he experienced his great love. He meets Jack Firebrace, a tunneller who ends up helping him endure the ravages of war and enables him to make peace with his feelings for Isabelle, who he is set to meet again.