Hex Hatfield calls McCoy, who is his employee and tells him to find the Thorned Emerald, then gives him a map. Using it, McCoy goes through pits and caves. While going, a black silhouette chases him but he doesn't realize. Then, he finds the emerald that his boss wants. The emerald is on a stone pedestal and "BEWARE THE CURSE" is written on the wall behind it, but he takes it and goes to sleep on a rock. While sleeping, he realizes that something is strange with himself and starts to become a walking cactus, as the sleeves of his clothes tear and thorns come out of his hat. Afterwards, a ghost figure comes to the dark sky and calls McCoy and tells him that his name is Popoca and he too was cursed by the Thorned Emerald. He also tells McCoy that if he doesn't take the emerald back to its place in time, he will become stone. McCoy gets aware of the situation and starts to go to the home of the emerald. The black silhouette, who is revealed to be a goon of Hatfield, sees this and tells it to him. Hatfield gets very angry, as he thinks that McCoy stole his emerald and his money and tells all of his goons, the Enemigos to catch "Cactus" McCoy.
The only playable charater is Cactus McCoy. While the arrow buttons make him walk and aim if he has any throwing weapon like sheriff badges or daggers, the A button makes him jump and the S button makes him punch or use his weapon if he has any. The game has 12 areas and each area has 5 challenges to do and 5 treasures to find, which are scattered in the area. When an Enemigo is killed, he drops his money and sometimes his weapon that can be taken and used when McCoy kneels on it. There are totally 59 types of weapons that McCoy can use in the entire game (including his punches). New types of weapons are introduced in all of the areas except Emerald Shrine, which is the 12th and the last area of the game. Each weapon is different than another in its power (darts and bazooka, for example). The money might be a bronze coin, a silver coin, a golden coin, a ruby, an emerald or a diamond. The values of them in order are 1, 5, 10, 20, 30 and 50. Money is used to upgrade McCoy on punching, swinging, shooting, throwing and whipping up to level 6 after finishing an area or in the menu of the game.
At the end of each area, McCoy opens a wide chest and one of the pieces of the map for the home of the emerald comes out.
The scoring depends on how many of the challenges are done (500 points for each challenge done - if all 5 challenges are done, then 500 * 5 = 2500 points), how many of the treasures found (500 points for each treasure found), the amount of money gotten , earned badges (each badge has a point between 150 and 750.) and the hit points. The hit points are earned by hitting an Enemigo (10 points for each hit) and juggling him while he is dying (the point for each hit depends on the type of the Enemigo).
- Cactus Canyon
- Prospector Mines
- Rustler Railroad
- Shady Springs
- Powderkeg Pass
- Midnight Express
- Rattler Ravine
- Brimstone Mine
- Deadridge Railway
- Sunset Gulch
- Emerald Temple
- Emerald Shrine
The flash game generally received positive reviews from the critics. The game's reviewer website JayIsGames.com said that the game has the depth and elements that can only be found in paid-for titles. According to the votes of the players in the same website, the game currently has a score of 4.7 out of 5.
- The concept of Cactus McCoy's fight against Hex Hatfield appears to be based on the real-life Hatfield–McCoy feud fought between the Hatfield family and the McCoy family. This being mainly their names, that it is set in 'Western' times, and it being a feud between the two.
- This is the only Cactus McCoy game so far to have one boss (who is Hex Hatfield).
- Perhaps due to the obvious usage of so many types of weapons against humans by Cactus McCoy, this game is rated as "Teen" in Newgrounds.com. However, the second game is rated as "Everyone" in the same website, even though it kept the same tradition as the first one and even added thrusting weapons to the arsenal.
- This is probably the first and the last Cactus McCoy game to use the old animation of Flipline Studios at the beginning.
- The Papa's games (another series made by Flipline) makes multiple references to Cactus McCoy and The Curse of Thorns, mostly through cameo appearances.