From the Super Mario Wiki
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Mario starts off on a platform next to some spikes. Donkey Kong throws Poison Mushrooms down from his perch throughout, which Mario must avoid. The platform on the left leads to a set of beanstalks, with a moving platform that leads to another beanstalk and the next section. This section contains some fruit and a Snapjaw, with a platform near where Pauline is standing; reaching her will clear the level, with Donkey Kong taking her to the next world.
In the cutscene that follow, Mario comes towards Donkey Kong and Donkey Kong Jr. with a hammer, but the pair are on higher ground; Mario throws his hammer upwards, climbs the ladder, and reobtains it, with Donkey Kong Jr. and Donkey Kong running away as he gives chase.