Amu is our main character. She wished for the courage to become her would be self. When she woke up the next morning there were three eggs (Shugo Chara's) in her bed. She took the eggs to school, where the boy she like whom everyone calls "Prince" wanted to talk to her. But since she pretends to have a different character on the outside she said that she didn't care even though she really did. As she was walking away from the "Prince" (who's name is Tadase) he noticed along with another one of his friends (Nadeshiko) that she had Shugo Chara's. When there was a school meeting and the guardians (who are practically the people who take care of things in the school there are four of them: King's Chair: Tadase, Queen's Chair: Nadeshiko, Jack's Chair: Kukai, Ace's Chair: Yaya) are giving the announcements Amu does a Character Change (which is when someone who has a Shugo Chara changes to the personality of that Shugo Chara) on accident not really knowing what it is and confesses her feelings for Tadase. After his rejection she runs out wishing she could just fall in a hole, when she does. She ends up meeting her first egg, whose name is Ran, she is Amulet Heart.
Amu does character change again, still not knowing what it is, and flies out of the hole. She lands on top of a construction site and has turned back into her regular self so she has no means of getting down. There a boy whose name is Ikuto who had smelled her eggs appears. He has cat ears and a tail, and is also followed around by a Shugo Chara whose name is Yoru. He succeeds in getting her eggs but she won't let him have them so she goes after them. He drops them on accident and she jumps off of the construction site to get them. Ikuto also drops something that he stole called the Humpty Lock, which he has the matching pair for called the Dumpty Key. As Amu is falling she grabs the Humpty Lock and does a Character Transformation (this is when you become your Shugo Chara, you transform into them) into Ran. Ikuto is amazed that she can do one. At first she is a little scared, but then she realizes that she is flying and she likes it. This is until she stops believing in herself and she changes back. Tadase had run after her and had seen her Character Transform. You find out that he also has a Shugo Chara, and he does a Character Change and uses his power to save her from the fall. He tells Ikuto that taking the Humpty Lock is unacceptable, but after looking away for a moment Ikuto is gone.
Amu later has to draw in a class of hers. She ends up doing it well because she did another Character Change but this time it was with her second Shugo Chara named Miki. She has to catch up to Miki and eventually they get to her and she comes along. Miki is very good at art, and she is Amulet Spade. Next when she wants to make something for Tadase, she gets help from Su her third Shugo Chara, who is really good at cooking. She is Amulet Clover. She accidentally calls Tadase "Prince" out loud which causes him to do a Character Change. Amu finds out that Tadase's dream is to rule the world so whenever he gets called "Prince" he becomes his Shugo Chara whose name is Kiseki, for a little while. Amu finds out that Tadase also wished to have the courage to be his would be self, and she feels they are alike.
All of the guardians explain to Amu what Shugo Chara's are. They are your would be selves, the people you wish to be, or what you wish to be. They want her to be the fifth guardian, the Joker. At first she doesn't accept it. Later she is confronted with an X Egg (which is when the owners feel they can't be who they really want to and their hearts eggs turn to their doubts). The X Egg turns into an X Character (it is when the X Egg opens and it is small like a Shugo Chara and it's the persons feelings that are having doubts). She Character Transforms and opens her own heart to return the hearts egg to normal. After this she realizes she wants to help more people and stop all the X Eggs and X Characters. She takes the position of the Joker. She does wonder why she was given the Jokers position, and Kukai explains she is the only one with three Shugo Chara's, so she is the trump card like the Joker.
Every Child has an egg in their heart, unseen at first, an egg with their dream. Hinamori Amu part of a group called the guardians, protect those eggs from becoming x-eggs, making sure everyone can hold on to there dreams. Amu, is the Joker Chair. The different chairs are King's chair, Queen's chair, Ace's Chair, Jack's Chair and the Joker's Chair. Amu's job is to capture and restore x-egg to there original form. She does this with the help of her three shugo chara (guardian characters or her would be selves), Ran, Miki and Su. She is the only guardian that can character change. Which is your typical magical girl transformation. Just like in all magical girl series there is romance too. She may say she loves Tadese (king), but she also likes Ikuto. Too bad that there are on opposite sides. Ikuto works for a company called Easter. Both sides, Gardians and Easter are looking for something called the embryo.