Plot

A boy with an active imagination faces his fears on an unforgettable journey through the night with his new friend: a giant, smiling creature named Dark.

Orion seems a lot like your average elementary school kid shy, unassuming, harboring a secret crush. But underneath his seemingly normal exterior, Orion is a ball of adolescent anxiety, completely consumed by irrational fears of bees, dogs, the ocean, cell phone waves, murderous gutter clowns, and even falling off of a cliff. But of all his fears, the thing he's the most afraid of is what he confronts on a nightly basis: the dark. So when the literal embodiment of his worst fear pays a visit, Dark whisks Orion away on a roller coaster ride around the world to prove there is nothing to be afraid of in the night. As the unlikely pair grows closer, Orion must decide if he can learn to accept the unknown to stop letting fear control his life and finally embrace the joy of living.

Orion (Jacob Tremblay) is a severely anxious 11-year-old young schoolboy with an endless list of irrational fears (Clogging the toilet and flooding the whole entire school, giving the wrong answer in class, cellphone waves giving him cancer, mosquito bites getting infected, falling off a skyscraper, being responsible for his team losing, locker rooms, being punched by school bully Richie Panichi (Jack Fisher), his parents (mom (Carla Gugino) and dad (Matt Dellapina)) moving away while he is at school). He journals his fears (like murderous clowns, bees, dogs, oceans) in his diary and is nervous about being rejected by his school crush, Sally (Shino Nakamichi), at an upcoming planetarium field trip. Orion wants to talk to Sally but cannot even approach her due to the fear of rejection and humiliation. So, he has imaginary conversations with her in his head. Orion is so afraid, that he tells the teacher that he did not get permission from his parents to go on the field trip. Orion's parents understand his fears and advise him not to let his fears get in the way of living his life.

One night, after a sudden blackout, Orion is greeted by Dark (Paul Walter Hauser), the embodiment of his worst fear, in his bedroom. Tired of Orion's constant complaints about him, Dark offers to take Orion on a trip to help him overcome his fears by showing him the benefits and wonders of nighttime. This way Orion won't be terrified of the Dark all the time. Dark says that he has been around for 500 million years, ever since life on Earth developed light sensitive proteins in their bodies. Dark is dejected that everyone hates him and thinks that he is evil.

During their travels, Dark introduces Orion to fellow night entities Sleep (Natasia Demetriou), Insomnia (Nat Faxon), Quiet (Aparna Nancherla), Unexplained Noises (Golda Rosheuvel) and Sweet Dreams (Angela Bassett). Dark convinces them to let Orion witness their work, to which they reluctantly agree. Dark argues that he cannot go back to put the kid back in his home, so the kid is stuck with Dark for 24 hours. And if Orion is cured of his irrational fears, it would mean less interference in all of their work. As they travel, Dark shows Orion how Quiet removes surrounding sounds, Sleep induces people to slumber (using a magic pillow, a chloroform filled handkerchief and a sleep hammer), Insomnia induces anxiety and wakes some people up (he claims that he is just the megaphone for the thoughts people already have), Unexplained Noises makes various noises outside homes, and Sweet Dreams generates wonderful dreams (Orion realizes that anything he thinks or says while in someone else's dream can make it come true).

Orion gradually realizes that now he is not afraid of the Dark. Initially, Orion's anxious behavior interferes with the night entities' jobs, but as Orion warms up to and eventually befriends Dark, he helps the night entities in their tasks. Additionally, Orion briefly encounters Light (Ike Barinholtz), Dark's nemesis who brings daylight in the mornings while Dark brings nighttime in the evenings.

Continuing their journey, Orion inadvertently remarks how Light seems preferable to Dark, as Light makes him feel safe and warm. Orion makes the case to the other entities that in a way, Dark is the problem. Feeling dejected, the other entities abandon their nighttime duties in exchange for working during the day. The other entities are now convinced that it is because of Dark that everybody hates them too, and they are sick of living in Dark's shadow. Angry and saddened by their abandonment, Dark stops on a mountaintop. A guilty Orion pleads with Dark to move before Light passes through and disintegrates him but Dark stays still and vanishes as Light passes through him, abandoning Orion. Now alone, Orion sits on the mountaintop, which he realizes is the back of a flying turtle, ashamed of his actions.

With the story ended, it is revealed that an adult Orion (Colin Hanks) was telling the story to his young daughter, Hypatia (Mia Akemi Brown), to help with her fears. As they walk through the city to the planetarium (where adult Orion now works), Hypatia is shocked by his ending and suggests a different narrative. As she takes over the story, Orion, now alone at the beach, is met by Hypatia (both Orion and Hypatia are kids in this version of the story) who promises to help him. She recites a poem she has written based on the story so far and the night entities return, having witnessed the chaos brought by endless daylight without nighttime to maintain the natural balance of the world. Without Dark, people start missing the night and can no longer see the stars. Remembering Dark to be the literal embodiment of Orion's worst fear, they realize Orion needs to sleep and dream about Dark to bring him back.

With Sweet Dreams' help, the two children enter Orion's subconscious and successfully summon Dark from the memory of the first time they met in Orion's bedroom, but the reunion is cut short when his closet door opens and reveals a black hole which tries to pull Dark in. Finally learning to let go of his fears, Orion jumps in to save Dark, while Quiet softly wakes him up right in time for them to get out of the dream. Dark reappears, restoring the natural order of the world, and returns the children to Orion's house before bidding them farewell. After Dark's departure, Hypatia is now stuck 20 years in her past with no way to return home. However, the story is resolved as a young boy named Tycho (Nick Kishiyama) arrives in a time machine to bring Hypatia back.

The story ends again, and it is revealed that an adult Hypatia (Shannon Chan-Kent) is telling the story to her son, Tycho. Finishing the story, Hypatia goes outside to say goodnight to her father and mother, the now much older Orion and Sally, as the scene cuts back to the start with a young Orion and Sally gazing at the stars on the Planetarium field trip.