Borys “Kosa” Kosiński (Eryk Lubos) is an MMA fighter at his peak. In the most important fight with his biggest rival, Deni Takaev (Mamed Khalidov), he makes a mistake that costs him his career and loses everything. Underdog tells a story about human weakness and strength, about despair overcome by budding love, about fighting with fate and rising after a fall, about sacrifice, family and friendship.