Paddington, the beloved bear voiced by Ben Whishaw, has seamlessly integrated into the Brown family household, which includes his adoptive parents Henry Brown and Mary Brown, along with his siblings Judy Brown and Jonathan Brown. Living happily in Windsor Gardens, Paddington has become a cherished member of the community, offering warmth and support to those around him.
To celebrate Aunt Lucy's (voiced by Imelda Staunton) 100th birthday, Paddington decides to buy a special pop-up book of London from the antique shop run by Samuel Gruber. This book holds sentimental value as Aunt Lucy rescued him when he was just a cub and sacrificed her dreams of visiting London to raise him. To save up for the book, Paddington takes on various odd jobs, but his excitement quickly turns to despair when the precious item is stolen.
After witnessing the heist, Paddington pursues the thief but is ultimately framed and arrested. Despite the disbelief of both Mr. Gruber and the Browns regarding his guilt, Paddington finds himself wrongfully convicted due to a lack of evidence. The true culprit emerges as Phoenix Buchanan, a self-absorbed actor living across the street from the Browns, who is after the book for its hidden treasures.
While incarcerated, Paddington befriends fellow inmates, including the grumpy chef Nuckles, who is impressed by Paddington's famous marmalade sandwich recipe. Meanwhile, the Browns are tirelessly working to exonerate him, putting together sketches in hopes of identifying the real thief. They enlist the help of a fortune teller who reveals that the stolen book is a key to a fortune belonging to its original author—an artist wrapped in mystery and tragedy, tied to a magician desperate to claim the treasure.
As Paddington's adventurous spirit brightens up the dreary prison life, he struggles against hopelessness as the Browns search fruitlessly for proof of his innocence. Convinced that Buchanan is indeed the mastermind behind the crime, the Browns initiate their own covert operation inside his home. They stumble upon a secret attic filled with costumes and the precious pop-up book, but their plans are interrupted when Buchanan returns home unexpectedly.
Feeling abandoned after missing a visit from the Browns, Paddington resorts to plotting an escape alongside Nuckles and other inmates, who surprisingly abandon him in pursuit of their own freedom. Ultimately, Paddington bravely reaches out to the Browns, who assure him that Buchanan is the true thief. To confront him, they set a plan in motion at Paddington Station, where they suspect a train carrying the treasure is set to depart.
Just in the nick of time, Paddington leaps onto the moving train, with the Browns hot on his tail just across the platform. However, a chaotic confrontation ensues, leading to Paddington being trapped in a carriage that is unceremoniously severed from the train and thrown into a nearby river. As Mary Brown battles to save him with the aid of Nuckles and other inmates, they redeem themselves and come together to assist Paddington.
In an exhilarating climax, Henry Brown manages to incapacitate Buchanan, who is promptly apprehended. Paddington, now vindicated but weakened from the ordeal, experiences a brief coma but awakens just in time to celebrate Aunt Lucy’s special day. Although disappointed that he couldn’t secure a gift for her, he is overjoyed to discover that the community has banded together to bring Aunt Lucy to London as a surprise.
In a heartwarming reunion, Paddington rushes to embrace her, wishing her a joyous birthday. Meanwhile, Buchanan faces justice with a hefty sentence and begins entertaining fellow inmates, putting his theatrical talents to use behind bars.
