Toramizu Ginta is a 14-year-old boy who is always dreaming about living in a world where trees talk and where there are all kinds of creatures and everything can come true. Unfortunately his everyday life is really different from what he dreams,in fact his classmates make fun of him and obviously they don't believe what he says, everybody but his childhood friend Koyuki. This lovely girl really believes Ginta,and she also asks him to take her with him once he'll be able to go there.
The very next day,at school,Ginta finds in front of him a huge 'dimensional door' (that is an ÄRM) which invites the boy to enter an other world because some mysterious person needs him there with urgency. Ginta leaves without any doubts before the amazed eyes of his classmates and since Koyuki can't go with him he promises her that he will tell her everything once he gets back.
This way he enters in MÄR Heaven. This new world is the one he has always dreamt about, and there he'll meet all the people that will be his companions during his stay: the witch Dorothy, the ÄRM Babbo who is able to talk, the princess Snow (who looks the same as his beloved friend Koyuki) with her faithful servant Ed, the mysterious bishounen Alviss with his little fairy Bell, the good old Aran and the playboy Nanashi.
That's how his great adventure begins. In this world he acquires all the skills he has never had: he's strong, robust, powerful... he makes a lot of friends and with them he'll start the fight against the Chess no Kouma and their leader Phantom, who want to destroy the world that Ginta loves and wants to protect.