Minutes after going into the referee's book, Paddy McNair makes a good break down to the Birmingham byline.
Just when he tries to pull the ball back though and cross it into the box, Krystian Bielik blocks it.
Dan Barlaser steps up and while Millwall keeper George Long goes the right way he's got no chance of stopping his strike.
That's Barlaser's first goal of the season and the perfect start for new Rotherham boss Matt Taylor.
There was a period of reflection around the grounds before kick-off, to remember those that died in a crush at an Indonesian football match over the weekend.