Gabi depicts the assassination attempt of King Gojong (Park Hee-Soon) - the 26th king of the Joseon Dynasty. Set towards the end of the 19th century, when coffee was first introduced to Korea, Russian sharpshooter Illichi (Joo Jin-Mo) and Joseon's first barista Danya (Kim So-Yeon) are manipulated by Japanese officer Sadako (Yoo-Sun) into an elaborate plan to assassinate King Gojong.
Danya and Illichi are lovers who work together in Russian swindling gold and coffee from their unsuspecting victims. The duo are captured by Russian soldiers and eventually handed over to Japanese officer Sadoko (Yoo-Sun). Sadako then recruits Illichi and Danya into operation Gabi. The goal is to assassinate Korea's Emperor Gojong. The penalty for their failure is death.
Meanwhile, King Gojong hides out on the premises of a Russian legation after his wife is killed by Japanese assassins. King Gojong works on staging the future Korean Empire.