Howl is a very skilled and very powerful wizard whose only goal is to maintain his freedom and is very handsome and vain. He is also a bit of a coward. Many believe he likes to steal the hearts of beautiful girls, but this is a lie.
At the beginnig of the film, Sophie is being harassed by two soldiers until Howl shows up and uses his magic to make the two march off and leave. He escorts Sophie to the bakery her sister works at and warns her he is being followed by slime creatures. The two evade them and Howl gets Sophie to the bakery safely. The slimes creatures work for the Witch of the Waste who inform her of Sophie. When Sophie closes the shop for the night, the Witch of the Waster curses her into looking like an old woman. Sophie leaves town for the Wastes to find the Witch until she reaches Howl's castle. Calcipher, the fire demon that works for Howl proposes that if Sophie can break the spell that makes him serve Howl, he would break the spell on her. To stay in the castle, Sophie says Calcipher hired her as a cleaning lady and begins cleaning the castle. After Sophie organizes the bathroom, Howl accidentally turns his hair orange and then black. He summons the Spirits of Darkness (which he did once before when a girl dumped him.) Sophie, seeing how miserable Howl is that he's "no longer beautiful" is reminded of her own low self-esteem that she was "never beautiful" and runs out of the castle to cry. She calms down and gets Howl cleaned up.
Howl says he has to report to the King and admits he hates how much of a coward he is and asks Sophie to go to castle and pretend she is his mother. Sophie goes and meets Madame Suliman who has removed the Witch of the Waste's powers and made her into an old woman. Suliman explains that Howl's heart was consumed by a demon and made him selfish. After hearing how Suliman plans to remove to Howl's powers if he does not use his powers to aid the King, Sophie stands up to Suliman and her confidence temprarily removes the age curse. Howl shows up and helps Sophie and the Witch of the Waste escape, along with Suliman's dog Heen.
During a dream, Sophie sees Howl transformed into a bird monster and she admits she loves him. Sophie wakes up and the next day, Howl uses Calcipher to change the locations the castle's magic door's lead to. They move next to Sophie's hat shop and Howl shows Sophie a beautiful field of flowers. Sophie, young asks Howl if he's leaving and says she wants to help "even though she's not pretty and all she's good for is cleaning." Howl says she beautiful but her low self-esteem renews her age curse.
The war soon hits the town and Howl saves Sophie from a bomb. He tells Sophie that he's no longer afraid and she is the person he wants to protect. Sophie, with her newfound bravery which breaks her curse permantly, takes Calcipher out of his hearth, removing the castle from the hat shop, and causes it to crumble. She gets back inside the remains and gives Calcipher her now silver hair and he gets the remains of the castle moving to reach Howl. The Witch of the Waste realizes Calchipher has Howl's heart and tries to get it only to be engulfed in flames. Sophies saves the Witch of the Waste by dumping water on her which also hits Calcipher almost killing him. Calcipher had said several times that if he died, so would Howl. Sophie cries, thinking Howl is dead until she sees the magic door and ends up in the memory of Howl's childhood. Calcipher is revealed to have once been a fallen star until Howl ate it and spit out Calcipher. She tells Howl and Calcipher to look for her in the future which is why Howl was able to find her in the beginning of the film.
She soon sees Howl in bird form and they fly to Calcipher. The Witch of the Waste reluctantly gives Calcipher to Sophie who puts him in Howl's chest getting his heart back. The last of the castle falls down a cliff but Turnip, a magic scarecrow, sacrifices himself to slow the descent. Sophie kisses Turnip as thanks, who is then revealed to be the Missing Prince of the neighboring kingdom and the reason for the war. The Witch of the Waste said that a kiss from his true love broke the spell on him, but since Sophie is in love with Howl, the Prince leaves to end the war. Sophie hugs Howl after he says her hair is like starlight. Howl and Sophie are last seen kissing on a repaired castle that is now flying.