After Mario had left to save the princess yet again, Luigi had been all alone with nothing to do but tend to the house. After going out to check on the mail, he finds a strange light glowing in the deep reaches of the forest around the house...

Luigi quickly investigates and finds a portal, which violently tears him from his dimension into a new one, Equestria.

After accepting Luna's deal, he now has to willingly defend the new world that is Equestria, and learn more about the wonders of friendship and himself...