Ah... the game that started it all and the reason the gaming industry is still alive. One of the best games of all time.
The plot is that a tribe of turtles called the Koopa Troopas have invaded the Mushroom Kingdom and have taken over the peaceful land. King Koopa has captured Princess Toadstool, the ruler, and taken her prisoner. An Italian plumber named Mario must go out to save her and the kingdom. This plot style with the princess being kidnapped would be a formula for all Mario games to come.
The gameplay was different than most games back then. It is a sidescrolling adventure that requires you to get to the end of the level and touch the flagpole to finish the level. Along the way, you can collect power-ups and kill enemies as well as find secrets. The enemies consist of goombas, koopas, plants, lakitus, spinies, and King Koopa. Except for spinies, King Koopa, and plants, each enemy can be killed by jumping on them. It's a simple game but it has a charm that can't be matched.
The power-ups are simple. If you get hit by an enemy, you die. The Super Mushroom lets you take another hit and makes you grow bigger. The fire flower lets you fire fireballs at enemies making it much easier to do so. What's a little disappointing about this game is that if you get hit when you have the flower, you go back to regular Mario instead of Super Mario, a trait that Nintendo would omit in future Mario games.
Super Mario Bros. is one of the best games ever and still to this day remains a piece of history and a really fun game.