NOTE: This spoiler was submitted by Jeremy
The film starts with grainy documented footage of the Du Pont family. They are with their horses and dogs on their piece of land, Foxcatcher Farms. They set loose a small fox on the farm, allowing it to run freely.
The year is 1987. Mark Schultz (Channing Tatum) is an Olympic wrestling champion. He is training in the gym with a dummy. Next, he goes to McKinley Elementary School to speak before an unimpressed audience of children about his accomplishments and what it means to him, showing off his gold medal. Mark lives alone in a small home. His older brother is another well-respected gold medal winner, Dave Schultz (Mark Ruffalo). The two practice together in the gym at Wexler University, resulting in Mark hitting Dave in the face and making him bleed.
Mark receives a phone call inviting him to fly out to Pennsylvania to meet John Du Pont (Steve Carell). Mark is brought to John's huge mansion and meets the man face-to-face. John asks Mark about his ambitions, to which Mark says he wants to be the best in the world. John wants to put together a wrestling team, aptly named "Team Foxcatcher". He also wants Mark to ask Dave to join them.
Mark goes back to Dave's home while he is outside with his wife Nancy (Sienna Miller) and their two kids. Mark invites Dave to join him and John in training their team, but Dave doesn't want to uproot his family and prefers to be settled where they currently are.
Mark moves out of his home and goes to live on John's estate. He is told that the horses on the farm are off limits as they belong to John's mother Jean (Vanessa Redgrave). John and Mark gather five other men to start their training. They all develop a familial bond, with the men treating John like a father figure, especially Mark. John has one of the team members move his mother's horse-breeding trophies off the shelf so that he can make a spot for wrestling trophies. John then falls over, and the men run to help him up, but it's just a trick to get one of them into a wrestling mood. The men cheer John's name.
The team goes to France for a wrestling competition. Mark competes and wins against his opponent.
John takes Mark around the country making rounds at events, photoshoots, and other gatherings to promote the start of Team Foxcatcher. Mark tells everyone that he sees John as a father figure that he never had. At one point, Mark brings John to Dave and Nancy's motel room to meet them. Nancy doesn't get up from the bed to greet John, making Mark upset. John also gets Mark to try cocaine, and the man even gets blonde highlights in his hair.
John sits on his porch with Mark. He asks Mark if he has any friends. He remembers a friend he had in his childhood, until he learned that he was paid by his mother to be his friend. He considers Mark a friend and tells him that he ought to call him "Eagle" or "Golden Eagle" instead of "Mr. Du Pont"
One day, John notices nobody is training in the gym. He finds the team with Mark at his place watching TV and smoking weed. John is unhappy with Mark making the call to give the guys the morning off. He slaps Mark and calls him an ungrateful ape. Mark is pissed.
Dave decides to move with his family to Pennsylvania to help his brother train with the team.
The team goes to Seoul for the XXIV Olympics. In Mark's first fight, he loses. He goes to his room and cries, then he pounds his head and smashes it against a mirror until he completely shatters. He then loads up heavily on carbs. Dave comes by his room and consoles him, saying he won't let him be alone. Mark pukes up his food and then gets weighed in (one of the weigh-in officials is played by the real Mark Schultz). He's 12 pounds overweight and has 90 minutes before the next match. Dave has Mark exercise heavily. John comes by the gym, but Dave talks him away from Mark. Later, Mark competes again and he wins.
John tries to show his mother what he is doing. She considers wrestling to be low, and she doesn't want to see her son be low. During a training session with the team, Jean's nurse brings her around the gym so John can show her what he's doing. She remains unimpressed and has the nurse wheel her away, to John's disappointment.
After another wrestling match, John is nowhere to be found. Dave learns that John flew home because his mother passed away.
John is the subject of a documentary that looks at his work behind Team Foxcatcher and his involvement with his team. Some footage of Mark is used. Dave is sat down to be interviewed for the film. He is told to describe John as a mentor and leader, though Dave seems very hesitant to say that.
Dave tries to negotiate with John and his lawyer over Mark's payment if he is to stay on the team. The lawyer says Dave is not quite qualified to be making these kinds of calls, but Dave affirms that as long as he is involved with the team, the demands ought to be met. Mark eventually moves out of John's estate.
On one cold day, John drives to Dave's home with his associate Fred Cole (Brett Rice). Dave is in the driveway working in his car. John pulls up and Dave walks over. John holds up a gun and asks Dave if he has a problem with him. A frightened Dave answers no, and John shoots him in the stomach. Nancy hears and runs to the door. Fred tells John to stop. John shoots Dave again in the back, and then points the gun at Nancy before she runs back inside to call 911. Dave starts crawling away, but John shoots him once more in the back, ultimately killing Dave. Fred gets out of the car as John drives away. He runs to Dave before Nancy comes to her husband's body.
The medics arrive and take Dave's body away. The police find John in his home and promptly arrest him.
A while later in the 90's, we're at a UFC fight. Mark is competing in it. It's never shown or stated, but he wins.
The ending text says that Dave Schultz was posthumously inducted into the Wrestling Hall of Fame and is survived by his wife and two children. Mark Schultz retired from wrestling after the Seoul Olympics and now operates a wrestling clinic. John Du Pont died in prison in December 2010.