The film starts with Sam Greenfield (Eva Noblezada) doing a music video with her friend and fellow foster kid Hazel (Adelynn Spoon), which gets ruined due to the fact that Sam has incredibly bad luck. Sam is 18 years old and has aged out of the foster care system, so she is now set to leave the home and Hazel to live and work on her own.

Sam stays in a small apartment where the pull-out bed traps her inside. When she wakes up the next morning, the windows to her apartment are open, and the leaves are blown inside. Sam tries to get ready for work, only for several mishaps to hold her back. She makes it to the store and meets her boss, Marvin (Lil Rel Howery). Sam makes a mess of things until Marvin finds something safe for her to do that won’t cause damage to the store.

Later, Sam buys herself a sandwich and sits down to eat alone, lamenting her constant bad luck. She is approached by a black cat and starts talking to him, even giving him half of her sandwich. The cat leaves, and Sam finds a penny with a four-leaf clover on it. Thinking about how she talked to Hazel about “lucky pennies,” she takes it with her.

The next morning, Sam finds that she now has better luck than before. She is able to do her job without worries, until she goes to use the bathroom and accidentally flushes the lucky penny down the toilet. Panicking, she runs back to where she found the cat and tries to find another penny. When she mentions the loss out loud, the cat, named Bob (Simon Pegg), starts speaking with a Scottish accent. Realizing his goof, he starts to walk away from Sam, but she follows him in the hopes that he may give her another lucky penny. After several mishaps, Sam finds Bob in an alley and watches him enter a portal to a new world. Sam hops in before it closes.

Sam finds Bob, who tells her she is in the Land of Luck, and humans are not supposed to be there. This world controls the flow of good and bad luck throughout the world, with many departments devoted to special types of luck. Bob tells Sam that if it is found out that he lost a lucky penny, he will be banished to the Department of Bad Luck. Sam wants to use the penny so she can bring it back to Hazel so she will be adopted. The two borrow clothing from Bob’s coworker Gerry (Colin O’Donoghue) to pretend that Sam is a tall leprechaun, and they use a button from her bag to pretend it is a penny so they can deposit it and get past The Captain (Whoopi Goldberg), who doesn’t like Bob and has it out for him.

Bob guides Sam to the Penny Depot where the leprechauns get pennies to borrow. Sam goes through the mechanism but ends up getting the button, and then accidentally redeposits it. Sam and Bob find Gerry and have to come clean about who Sam really is so he can help them. The Captain then finds out about the button, and then puts Bob alongside two other cats as suspects.

Gerry tells Sam and Bob about the bunny drones used to retrieve lucky pennies from the human world. They need a special luck crystal to power the drone. On the way to find one, the trio finds Babe the dragon (Jane Fonda), the luckiest being alive. Sam also learns of a machine called the Randomizer that sends good and bad luck throughout the world. It is powered by ancient luck stones, one green for good luck and the other purple for bad. The trio find the crystal and find a bunny drone to send out. Sam has to do an impromptu dance to “Lucky Star” to distract the bunny techs from seeing Bob controlling the drone in the human world. The drone retrieves a penny, but it somehow becomes liquified upon returning and gets sucked into a vent going to the In-Between, the space between Good and Bad Luck.

Sam and Bob take an elevator and accidentally end up in Bad Luck. Sam gets locked out of the elevator and has to sneak her way through the department to get to In-Between back with Bob. They find Jeff (Flula Borg), a unicorn that runs the department by working the Bad Luck Apparat that sends bad luck to the Randomizer. He tells the two that he sent the penny back into the deposit. The two also discover that Jeff used to be with Babe until he was moved down there.

Sam attempts to personally ask Babe for a penny. They have a heart-to-heart where Sam learns Babe still loves and misses Jeff, and Babe shares Sam’s belief that there should be more good luck than bad luck. She gives Sam a lucky penny to borrow so she can use it in time for Hazel’s potential parents to show up. Unfortunately, Bob gets caught by The Captain and is almost banished to Bad Luck until Sam uses the penny to save Bob and give it to The Captain, pretending to have misplaced it.

Still wanting to help Hazel, Sam and Bob turn off the Bad Luck Apparat to stop it from sending bad luck to the Randomizer. When it looks as though Hazel might get adopted, Sam feels confident enough to go home, but then the bad luck begins breaking through the machine until the Randomizer becomes damaged, causing bad luck to spread and remove all the luck from everywhere. All the clovers wilt and the pennies become unlucky, and Hazel’s potential parents never show up. Sam being a human is revealed to everyone, and Bob is also revealed to not be a lucky cat, he just had his own penny on him because he was an unlucky black cat (and also British, not Scottish).

Sam and Bob try to fix things by running down to Bad Luck, as Sam realizes you can still find good luck from bad luck. They meet the roots and goblins that make up the department, run by Rootie (John Ratzenberger). With their help, they are given good luck beads. Sam and Bob return to Babe with green and purple beads, allowing Babe to forge new stones. However, she forges two good luck stones in addition to a bad luck one, as she thinks there should only be good luck. Sam runs after her with the bad luck stone and convinces Babe that bad luck is still needed since it can still lead to good luck. Babe agrees and has Sam place the bad luck stone, turning everything back to normal.

Sam sees that Hazel meets a new couple that chooses to adopt her. Bob is offered an official position in the Land of Luck, but turns it down since he decides to stay with Sam. Jeff and Babe also resume their relationship.

One year later, Sam is doing much better at her job and has even been promoted. She lives with Bob and regularly spends time with Hazel and her new parents. Sam’s final narration states that bad luck led her to the best thing in her life.

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Sam Greenfield is terribly unlucky. She has aged out of the foster care system and has to live on her own, where she encounters several mishaps. Things change when she comes across a seemingly lucky black cat named Bob, who leaves behind a lucky penny that Sam grabs. While it briefly changes her luck, she loses it and tries to get a new one. She follows Bob and ends up in the Land of Luck, which controls the flow of good and bad luck in the world.

Sam and Bob try to get a lucky penny so that Sam can help her friend Hazel get adopted and so Bob will not get in trouble. After several misadventures, Sam helps Bob out of trouble and tries to help Hazel by shutting off the machine that distributes bad luck. This backfires when the bad luck overflows and destroys all the good luck. However, Sam and Bob fix things by running to the Department of Bad Luck, where the inhabitants have spare good luck. They bring it to Babe the dragon, who forges new luck stones to turn things back to normal.

Hazel gets adopted and Bob stays with Sam, and they all regularly spend time together as Sam embraces her luck.