A young boy and his little sister are playing Super Mario Bros. 3 on their Famicom, when their mother calls and tells them to come to her shop, however, by the time she is finished at work some time has gone by and the young boy is still playing Super Mario Bros. 3. His little sister gets impatient and leaves without him. She is too young to know where the shop is and she gets lost. The boy then runs off to look for his lost sister and almost gets hit by a driver, which was Mario. He gets out and gives him a small lecture on being safe when crossing the road.
Source: Super Mario Wiki