A Northern industrial town. Textile magnate Birnley remembers the crisis that recently beset his industry...

Birnley tours a mill owned by his daughter Daphne's fiancé, Michael Corland, who hopes to attract his investment. Birnley comes upon some curious equipment in a laboratory, which none of the technicians can explain. An investigation reveals an invoice for £4,000, and Sidney Stratton, who ordered the equipment without permission, is sacked.

Sidney takes a labourer's position at the Birnley mill, where he meets committed trade unionist Bertha. When the company takes delivery of an electron microscope, Sidney conveys it to the laboratory. Hoskins, the Head of Research, is impressed by his knowledge of the equipment and, taking him to be a technician, asks him to assist in the laboratory.

At his lodgings, Sidney informs fellow tenant Bertha and his landlady Mrs Watson that he has taken a new job without pay. Bertha is horrified and wants to involve the Works Committee, but Sidney begs her not to.

Birnley refuses to invest in Corland's mill and Daphne is furious. Leaving Birnley's office she spots Sidney and, recognising him from Corland's mill, delightedly rushes to tell her father. But Sidney intercepts her and explains his project. That night Daphne swots up on chemistry from her father's library.

Next day, Sidney has a breakthrough. In a frenzy of excitement, he rushes to see Mr Birnley, but is restrained and, when it emerges he has been working under false pretences, fired. That night, he visits Birnley's home, but the butler, Knudsen, won't admit him. Birnley is holding an emergency meeting with his shareholders to explain an £8,000 hole in the accounts.

Daphne lets Sidney in and runs to fetch her father. Meanwhile, Knudsen finds Sidney and tries to throw him out. Birnley arrives and demands Sidney leaves. Daphne berates her father, explaining that Sidney has invented an indestructible fibre which repels dirt. Birnley is dismissive at first, but later gives Sidney a new contract and an entire lab to himself.

Initial attempts to recreate the experiment are disastrous. With costs mounting, Birnley becomes concerned, but finally the experiment works. Production begins, and Sidney is fitted for a suit made from the fabric: in white, since it repels dye like dirt. Daphne hails him as a knight and hero of mankind.

Birnley is visited by a consortium of textiles manufacturers, led by Corland and the venerable Sir John Kierlaw. They are alarmed at the threat to the industry. As Sidney arrives, he meets Bertha and her union colleague Frank, who are similarly horrified as they realise the threat to jobs.

Sir John presents Sidney with a new contract, but when he realises their intention to suppress his invention he tries to leave. The bosses try to restrain him, but Sidney evades them. Finally he is knocked unconscious by a falling frieze.

Sidney is locked away while the bosses decide what to do. Money and reason having failed, they ask Daphne to seduce Sidney into co-operating. Disdainfully, Daphne agrees - but demands reward. Daphne offers to go away with Sidney if he accepts the money. Sidney is tempted, but refuses. Relieved, Daphne helps him escape.

Sidney plans to head for Manchester to speak to the press, but can't afford the train. He heads to his lodgings for money, and meets Bertha. When she realises his intentions, she locks him in.

The Birnley workforce is on strike. At Birnley's house, the strikers announce that Sidney is now in their hands. Meanwhile, Sidney escapes, with the help of a young girl. When they hear of this, workers and bosses unite to stop him.

Sidney is conspicuous in his white suit, and the mob is soon on his tail. Meanwhile, Wilson, working late in the lab, discovers the reels of fabric are disintegrating: the compound is unstable. Sidney is cornered, and the mob grabs at him; the suit comes apart in their hands. Sidney stands in his shirt and underwear, surrounded by laughter. Someone offers a coat, and he walks away, dejected.

Later, Birnley, grateful the crisis is over, watches Sidney leave the mill. As he walks, Sidney refers to his notes and, spotting the mistake in his formula, strides confidently away.