An American Tail

In 1885, Shostka, Russia, the Mousekewitzes, a Russian-Jewish family of mice who live with a human family named Moskowitz, are having a celebration of Hanukkah where Papa gives his hat to his son, Fievel, and tells of a wonderful place called America, where there are no cats. The celebration is interrupted when a battery of Cossacks ride through the village square in an arson attack and their cats likewise attack the village mice. As a result, the Moskowitz home, along with that of the Mousekewitzes, is destroyed.
In Hamburg, Germany, the Mousekewitzes board a tramp steamer headed for America. The crossing proves itself long and onerous, as well as the tramp steamer being buffeted by an angry sea. During a thunderstorm, Fievel suddenly finds himself separated from his family and washed overboard. Thinking that he has drowned, they proceed to New York City as planned, though they become depressed at his loss.
However, Fievel floats to America in a bottle and, after a pep talk from a French pigeon named Henri, embarks on a quest to find his family. He is waylaid by conman Warren T. Rat, who gains his trust and then sells him to a sweatshop. He escapes with Tony Toponi, a street-smart Italian mouse, and they join up with Bridget, an Irish mouse trying to rouse her fellow mice to stand up to the cats. When a gang of them called the Mott Street Maulers attacks a mouse marketplace, the immigrant mice learn that the tales of a cat-free country are not true.
Bridget takes Fievel and Tony to see Honest John, a drunk but reliable politician who knows all the voting mice in New York City. But, as the Mousekewitzes have not yet registered to vote, he can't help Fievel find them. Meanwhile, his older sister, Tanya, tells her gloomy parents she has a feeling that he is still alive, but they insist that it will eventually go away.
Led by the rich and powerful Gussie Mausheimer, the mice hold a rally to decide what to do about the cats. Warren is extorting them all for protection that he never provides. No one knows what to do about it, until Fievel whispers a plan to Gussie. Although his family also attends, they stand well in the back of the audience and cannot quite see that it is Fievel onstage with her.
The mice take over an abandoned museum on Chelsea Pier and begin constructing their plan. On the day of launch, Fievel gets lost and stumbles upon Warren's lair. He discovers that he is actually a cat in disguise, and the leader of the Maulers. They capture and imprison him, but his guard is a reluctant member of the gang, a goofy, soft-hearted cat named Tiger, who befriends and frees him.
Fievel races back to the pier with the cats chasing after him when Gussie orders the mice to release the secret weapon. A huge mechanical mouse, inspired by the bedtime tales Papa told to Fievel of the "Giant Mouse of Minsk", chases the cats down the pier and into the water. A tramp steamer bound for Hong Kong picks them up on its anchor and carries them away. However, the giant mouse contraption has caused the pier to catch fire, and the mice flee as human firemen, possibly the FDNY, arrive to battle it.
During the fire, Fievel is once again separated from his family and falls into despair when a group of orphans tell him that he should have given up looking for them. Papa overhears Bridget and Tony calling out to Fievel, but is sure that there may be another "Fievel" somewhere, until Mama finds his hat. Joined by Gussie, Tiger allows them to ride him in a final effort to find Fievel, in the end, the sound of Papa's violin leads him back into their arms. The journey ends with Henri taking everyone to see his newly completed projectâthe Statue of Liberty, which appears to smile and wink at Fievel and Tanya, and the Mouskewitzes' new life in America begins.

Question: Who gets lost and stumbles upon Warren's lair?
Answer: Fievel; Fievel