The main gist of the plot for the game is that you (Isaac) and your Mother are happy but then God gets involved and then it all goes down hill. This forces Isaac to escape into the sewers and so begins a brilliant indie game where you must fight of spiders, meat piles and the seven deadly sins in order meet the final boss, your Mother. As you go through each level you will collect bombs, coins and keys. Each of which can be used in their own ways. Throw each level having a special item, that can increase your ability, on each level and you have a great dynamic and yes fair Zelda like game. However, Zelda has nothing on the weird crazy world of BoI and I haven't play a game like it in a long time, something that I find very refreshing.
The basic controls are WSAD keys to move and arrow keys to shoot. Shift or E for bombs and space and Q for special abilities and items that are found along the way. Isaacs basic attacks increase as you play (or as you get items rather) making his 'tear' attacks do more damage, have a larger range and attack faster. Each character that you unlock has different attributes, this ranges from special abilities to health and movement speed. With lots of variety BoI is always a refreshing experience.
Unlockable characters, steam achievements, deals with the devil and challenging gameplay all make this a game worth playing. Something that I suggest you do.