Barely had the 18-year-old hit her hand on the tile after 3.56,26 before she hastened up from the pool, gave a "high five" to a leader, to prepare herself for the semifinal on 200 meter medley.
There she was also the fastest.
I already knew in advance that this would be my most important evening. So I'm really happy to win gold and then do a really fast and strong swimming on 200 meter medley (2.07,39). I've never done such a double before, so it promises well for the future, she says.
The winning time on 400 meter freestyle was over two seconds slower than her own world record from June at 3.54,18.
Despite being the world record holder, it was the first World Championship gold on the distance for the Canadian.
Li Bingjie, China, took silver and Katie Ledecky, USA, bronze.
Only Michael Phelps has won five individual gold medals in the same World Championship.
McIntosh also swims 800 meter freestyle, 400 meter medley and 200 meter butterfly in the World Championship. The final on 200 meter medley is decided on Monday.
Australia took a surprising relay double and won 4x100 meter freestyle on both the women's and men's side. On the women's side, the favorite USA was defeated and on the men's side, before Italy.
