Millie is a French narrow gauge tank engine, who lives and works on Sir Robert Norramby's private railway at Ulfstead Castle, where she helps out with maintenance or takes tourists around the castle grounds.
While Sir Robert Norramby was travelling the world, Millie was kept locked up inside her shed at Ulfstead Castle until he returned. After he came back to Sodor, the Earl brought Millie out of her shed, and she helped him with the reconstruction of Ulfstead Castle. (TVS; King of the Railway)
Millie met Luke for the first time when he brought a train of stone to Ulfstead Castle, and as she went to show him where to take the train, she argued with him about which job was better: working in the loud, dusty quarry, or quietly working at the peaceful castle grounds. They decided to swap jobs, and Millie went to the Blue Mountain Quarry to do Luke's work. Though she struggled at the beginning, Millie became a hard worker and ended up loving the Blue Mountain Quarry, but she was pleased to return back to the castle and do her usual work. (TVS; The Switch)
Millie assisted in the construction of the dinosaur park on the Earl's estate, and worked alongside Samson and Harvey. When Millie became frightened by a model dinosaur head that nearly hit her, Samson and Harvey teased her for being a silly little engine. In order to get back at them, Millie had the groundskeeper light a fire behind the volcano model to create the illusion of lava, frightening the two bigger engines. The Earl scolded Millie for this, but forgave her and even decided to modify the volcano model to actually emit smoke. Millie took the Fat Controller and his mother around the dinosaur park when they visited. (TVS; Millie and the Volcano)