Patada de Mula, an old retired boxer, lives with the guilt of having killed his opponent in his last fight, but he works as a masseur in a gym, sells balloons on Sundays and with the proceeds supports the widow of the opponent he killed and his son El Kid, an aspiring boxer, without his knowledge. Patada is also in love with his neighbor Marga, a young woman who is pushed into prostitution by the gangster Duque and his own father, but to whom Patada sends letters as an anonymous lover. The Kid is sponsored by Duke and detests Patada for the death of his father, but when he throws him out of the inn that his mother tends she confesses to him what he has done for them and the Kid asks her forgiveness and makes him his second.