"Underdog" is a story inspired by real events. Based on them, a script by Mariusz Kuczewski was created. Boris "Kosa" Kosiński (Eryk Lubos) is a MMA fighter who is at the top of the form. In the most important fight with his biggest rival Deni Takaev (Mamed Khalidov), he makes a mistake that negates his career. He loses everything.

"Underdog" is a story about human weaknesses and strength, doubt, which overcomes the nascent love, about the fight against adversities and rising after the fall, sacrifice, family and friendship.