NOTE: This spoiler was submitted by David G

Costa Rica, 2015: The Adventure Racing World Championship begins. Commentary from Bear Grylls explains that Michael Light (Mark Wahlberg) is one of the best adventure racers never to win a championship. Michael goes against the advice of team member Leo (Simu Liu) and leads the team out on their kayaks despite other teams waiting for a change of tide. When the group gets stuck in the mud, Michael’s determination to keep going frustrates Leo and Helen (Juliet Rylance), who eventually force him to call for help to support an injured fourth team member. Leo posts a video to social media showing Michael’s stubbornness.

Dominican Republic, 2018: Arthur, a stray dog, wanders the city streets scavenging for food. In Colorado, Michael continues to train for endurance racing. He lives an everyday life at home with Helen, his wife, and their daughter Ruby (Cece Valentina). He unhappily works as a realtor alongside his father, Charlie (Paul Guilfoyle). Michael and Charlie fight about Michael’s desire to compete once more. Michael is upset that his legacy has become Leo’s post, which went viral and ended his career; Helen encourages Michael to try and make some connections with potential teammates and sponsors to help him race again.

Michael travel to Big Sur to meet with Chik (Ali Suliman), an older former champion who stopped competing due to a bad knee, and to Hawaii to meet Olivia Baker (Nathalie Emmanuel), a professional climber and retired racer. He convinces them both to join his team. Michael meets with the executive team at Broadrail, a corporate sponsor, and tries to persuade them to support his team; when they question his determination, Michael makes an impassioned speech about how he will win no matter what and storms out. The Broadrail team agrees to finance the team under the stipulation that Leo becomes the team’s fourth member. Michael visits Leo on the set of a photoshoot for a product Leo, now an influencer, is promoting. Leo makes it clear that he knows Michael only wants him because Broadrail has insisted, and he asks Michael to acknowledge his leadership mistakes during the last race; Michael apologizes, and Leo agrees to join.

The team arrives in the Dominican Republic four days before the race with far less time to train due to limited funding. While practicing, they encounter their rival Decker (Rob Collins), and the teams playfully tease each other. The teams gather for a race kickoff celebration, and Arthur sits among the athletes in the crowd. The next day, Arthur encounters two other wild dogs who chase him through the city’s slums; he outruns them and escapes by jumping onto the back of a truck.

At the starting line, Olivia reveals she is racing for her father, who has been diagnosed with cancer, and the team hypes themselves up and strategizes for the five days ahead. The race begins on foot with teams running through a jungle trek; Michael immediately questions Chik’s navigation when another team branches off, but Chik pushes back. Back at home, Helen and Ruby track Michael’s progress online. The team arrives at the first rest point in 17th place; while resting, Michael encounters Arthur and offers him a meatball before the team heads off. The team begins the second and third stages of the race, which consist of all-terrain cycling; they bond and joke together as they ride. They attempt to take a shortcut to make up some time and are forced to climb a vertical rockface while carrying their bikes on their backs; despite the struggle, they make it to the top and find an abandoned zipline they intend to use to fly across a giant ravine, all while their bikes hang from their waists. Chik and Leo make it across successfully, but Olivia becomes stuck in the middle when one of the wires breaks. Michael goes out to help her, and the two manage to make it the rest of the way across after Michael comes up with a very risky solution that forces Olivia to detach and swing below Michael and his bike.

The team continues and makes it to the next rest point, now in 11th place; they continue to excel over the following two stages, climbing to 6th place when they arrive at the halfway rest point, less than an hour behind the leading teams. Michael convinces the team to rest for only 35 minutes instead of the suggested hour so that they can head out with the other top competitors, including Decker. The teams set out at night in the pouring rain; Chik tweaks his knee after tripping over a branch, and Michael offers to carry his pack to help him out. Leo begins filming the team during a short rest until Michael takes his phone and throws it into the jungle; Leo gets angry at Michael for letting them race despite knowing about Chik’s prior knee injury. Arthur shows up in the middle of their argument, and the team recognizes him from their earlier interaction. The team sets out again, and Arthur follows along, pushing Chik when he starts to slow down.

The team becomes lost after passing through a small town. As they try to navigate through the jungle, Arthur proceeds to get in front of them and bark loudly, alerting them and stopping the team from walking directly off a cliff.  The team is amazed that Arthur was able to help them, and they promise to give him more food at their next stop.  The team begins to cross a river, and Arthur swims away; Michael is reluctant to leave, though they collectively decide to move on. Arthur quickly returns to them, satisfying everyone.

Arthur continues to accompany the team as they move on to the following stages. Leo gets dizzy and begins throwing up during a hike up a steep hill. Michael wonders what Arthur’s life was like before they met him and shares some of their food with the dog, deciding to name him Arthur the King. Leo’s hydration continues to worsen, and the team makes use of a local fishing boat to help them cross another river and bring them to the next stop.

The team arrives and learns that they are now in first place; Leo is insistent he can keep going, but Michael decides that they will make good use of their lead and that the team will take a more extended rest so that Leo and Chik can recover. Michael speaks to a race supervisor to try and get Arthur care, but he is told that there are no vets. While Leo and Chik rest, Olivia talks to Michael about her father’s cancer diagnosis and how he pushed her to do the race in his honor.  Chik asks Michael why they only had five days to train; Michael admits that they could not afford longer and that he paid for half the team’s finances with his wife’s savings. As the team prepares to leave, Decker’s team shows up and mocks Michael’s team for their injuries and Arthur’s presence; the group surmises that Decker is hurting and needs rest, so they gather their gear and get back on the course with Arthur.

Helen reads news coverage and social media and learns about Arthur becoming the fifth member of Team Broadrail, excitedly sharing the news with Ruby. The final leg of the race begins with Decker’s team very close behind Michael’s as they arrive at the start of a kayak stage. Race organizers refuse to let Arthur join the group in the kayaks due to safety issues, and Decker’s team passes them as they argue. Michael reluctantly leaves Arthur behind despite Arthur’s whimpering. The team starts to paddle out and nearly overtakes Decker. Still, Arthur continues to follow, swimming toward them in the water and almost drowning. Michael decides to give up their chance of winning to turn around and rescue Arthur; they arrive just in time, and everyone acknowledges Arthur’s life matters more than winning.

The team continues the kayak for the next four hours as Arthur’s health begins to deteriorate. They arrive on land and begin the final sprint with Michael carrying Arthur on his back; they near the finish line and Arthur gets down and runs alongside Michael, Leo, Chik, and Olivia, as they ultimately finish in second place. The foursome celebrates and embrace, deciding that they already feel like winners.

Michael takes Arthur to a local vet who explains that Arthur is suffering from wounds incurred from prior abuse and they have become infected and infested by parasites; the doctor suggests they put Arthur down as they cannot perform the surgery required. Michael is insistent that he takes it back to the US, but he is told border agents would not allow him to enter the country due to his issues. Michael calls Helen and talks to her about how Arthur; Helen says Arthur is a fighter just like Michael. Michael prepares to say goodbye to Arthur, then changes his mind at the last minute and takes Arthur away. Arthur refuses to enter Michael’s hotel, and the two end up sleeping on the city street. Leo finds them the following day and takes them to the airport, where a large crowd has gathered to help support Michael’s effort to bring Arthur home, including Decker and all the competitor teams. Decker acknowledges the team would have won if they didn’t turn back and offers to run a future race with Chik.

Dominican customs officers allow Arthur to leave as the crowd cheers. Michael becomes emotional when the airport staff forces Arthur to get into a cage to board the plane, but Arthur willingly gets inside. Upon arrival in the US, Michael finds Arthur suffering and immobile in the crate, and he carries Arthur out, where they are met by Helen, who takes them immediately to the vet. In the waiting room, Michael learns that their story has become national news and that people have donated thousands of dollars to Arthur’s cause; Helen also reveals she is pregnant. Doctors perform surgery on Arthur but are not sure if he will survive; Michael goes to lie on the floor beside him and talks about their potential future together.

Eighteen months later, Arthur has fully recovered, and he and Michael run through the Colorado mountains together. Helen has given birth to a baby boy, and the family celebrates together. The epilogue text reveals that the crowdfunded money raised paid for all of Arthur’s medical expenses and started a foundation to support the well-being of animals around the world.