Role in the film
Clara is the middle child who was born into a very wealthy family. Unfortunately, her mother died at an unknown age, though it's known to have been years after she had left the Four Realms. While opening presents at home on Christmas Eve with her family, she receives a posthumous present from her mother, which happens to be a golden egg locked with a pin tumbler's lock (a lock that cannot be opened without a specific key.)
Clara's father tells her not to "disappear off into the night", but Clara disobeys him and visits her godfather, Drosselmeyer, and helps him repair a music box. They talk about Clara's mom and her death, and about how the Stahlbaum family is coping. She then returns to the party after getting helpful advice and a bit of history about her mother. However, her father is absolutely furious, and they have a short argument about the importance of maintaining their family's social status and Clara's mother, causing her to storm off.
Soon, Drosselmeyer comes out of his workshop and comes out and announces to all the children to find their gifts. Each guest's present must be found by following a red thread attached to a Christmas tree, which lead to various different locations around the estate. Clara returns, and follows the only golden thread. As she follows her thread, she is greeted by her younger brother Fritz, who's already found his gift, a nutcracker. She laughs at his antics, before going up a grand staircase and down a dark red hall. The pattern of owls on the wall soon turn into a pattern of rats.
She follows the thread into the hall, which soon turns into a log. She goes to the end of the log, in which she follows the line of string through a twisted thicket of white trees, in which she finds the key to the egg inside a decorated Christmas tree. As she is about to grab the key, it is snatched by a mouse. Angered, she chases the mouse as it hops across a frozen river that she is unable to cross due to the thin ice and the risk of drowning.
Refusing to "be bested by a small mouse", Clara instead attempts to cross the river by crossing an abandoned-looking bridge, attempting to hop over the barrier. Doing this, she causes Phillip the Nutcracker to awaken, who then threatens her, asking her name. He mixes up her name at first, so she explains that she is the daughter of Marie Stahlbaum. Who, as it turns out, was formerly queen of the Four Realms.
After Phillip apologizes profusely for the misconception, he takes Clara to the unnamed center kingdom, where the council of Regents resides, including the regent rulers of the Realms, The Sugar Plum Fairy, Shiver, and Hawthorn. Clara is greeted by the regents, who are saddened after learning that Marie Stahlbaum is dead. Even though they considered it a tragedy, Sugar-Plum enlisted them to create a ballet/pageant to show Clara a trip around all of the four realms, excluding the Land of Amusements. Clara is then given her mother's prized dress, before attending a ballet that showed how her mother discovered all the realms.
Clara is then led to her mother's machine, which can only be unlocked by using her Mother's key. So with a new quest ahead instead of just opening an egg, Clara assembles a small army to take on Mother Ginger and goes head-on into the Fourth Realm. Even though it started off well at first, multiple rats started digging holes that sucked Clara and her fellow soldiers into the ground. Screaming, she arrives at Mother Ginger's giant version of herself, and is greeted by a group of terrifying clowns, that are able to detach their body parts.