The miner Karel Vondra is breaking records in mining for coal at the Žofie coal mine. He wants to excel in what he does and he is suitably proud of his efforts. His records and air of superiority generate resentment among his colleagues. Vondra's partner Šimek is not able to support him nor go against all the others, so he leaves for another local mine. No-one wants to work with Vondra so he has to accept minor tasks. At this time the political secretary Diviš begins working at Žofie who soon discovers the roots of the inadequacies which prevent the mine from fulfilling the plan. Diviš explains to the miners how important it is to use mechanization and to work together. He encourages Vondra towards collective sentiment. He lends him political literature and helps him improve the machine which will increase the efficiency of the mine. Vondra gradually changes his attitude towards his colleagues who, in turn, help him work the machine. Everyone then eagerly increases their efficiency, working towards the day the red flag will fly above Žofie, a sign that the work is going at full speed.
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