Tin Cup is a romantic, but funny movie about a former golfer with no motivation in life except to drink and be with his buddies. This changes when one day a woman steps into his life and changes his course of life. Roy "Tin Cup" McAvoy (Kevin Costner) use to play professional golf when he was younger. He is older now and just wants to run his driving range, and drink. Dr. Molly Griswold (Rene Russo) shows up and wants him to teach her the sport of golf. Roy shows interest in her, but David Simms (Don Johnson) is her boyfriend, who is an acquaintance of Roy's who knew him years ago.
Finding out that Molly is taking golf lessons from Roy, Simms hires Roy as his caddy for an upcoming golf tournament. Roy and Simms immediately begin to fight and trying to one up each other. Roy is confident that he could make the shot. Simms warns him that he will no longer be his caddy if he does it. Roy makes the shot no problem and gets fired.
The shot inspires Roy to try out for the U.S. Open. This also gives him the opportunity to get Molly to help him in clearing his head and improving his self esteem. He hires his friend Romeo Posar (Cheech Martin) to be his caddy, but Roy's reckless behavior causes him to quit. Roy barely qualifies for the tournament. Molly sees that when Roy bets his confidence goes up, so she encourages friendly wagers. His golf game improves.
Roy improves so much that in the finals it is him, Simms, and pro Peter Jacobsen (himself). Simms who always goes by percentages decides to play safe on the last shot. This ties Roy and Jacobsen. Roy tries several more times to get the shot, but has the same results. Finally on his last shot and last ball, he goes for it and makes it. He wins the tournament on a 12th shot.