Cell Games was a tournament created by Cell, in order to show his power to the world. One of Cell's other reasons for creating the Cell Games is so that he can recreate the World Martial Arts Tournament.
Cell gives an international announcement for the games on May 17, Age 767, and gives the fighters who want to participate 9 days to prepare. The Earth's Military tries to stop Cell before the tournament begins, but fails. Mr. Satan announces that he will defeat Cell at the Cell Games.
Nine days later, on May 26, the Z Fighters have done as much training as they possibly could do, except Goku and Gohan, who are convinced that Goku is enough of a match in his Full-Power Super Saiyan form. When the tournament commences, Goku decides he will be the first one to fight Cell.
However, the first challenger to arrive at the Cell Games is Mr. Satan (winner of the 24th World Martial Arts Tournament which the Z Fighters did not enter). He challenges Cell himself along with his students, Pirozhki and Caroni. Cell proves to be far superior to them as expected, and they are quickly disposed of, with Mr.Satan claiming he slipped off the ring. Vegeta is the first of the Z Fighters to arrive, and is soon followed by the rest.
Goku is the first Z warrior to challenge Cell, and after a bit of sparring as a warm up, Goku increases his Ki to maximum, which astounds the rest of the Z warriors. However, surprisingly, Goku's son Gohan expects more power from his father. After another bit of sparring, Cell forms a Kamehameha which threatens to destroy the Earth, until Goku diverts it away from the Earth, and then returns to the ring using his Instant Transmission, only to surprise Cell with an attack. Cell attacks with a flurry of Ki Blasts. After much fighting, Goku reaches his limit and nearly falls from the ring. Cell, tired of being confined to a small area, destroys the ring, and announces that the entire Earth is the ring now. Goku, desperate to end the conflict, forms a Super Kamehameha while in the sky, and makes Cell believe that Goku will fire the Super Kamehameha and risk the existence of the Earth in order to defeat him. Everyone is concerned that Goku will actually destroy the Earth, when Goku teleports at the last moment to a close proximity to Cell, and shoots the Kamehameha, blowing off Cell's head and upper body. Although Cell is able to regenerate from the attack due to having Piccolo's regenerative ability, it leaves both him and Goku significantly weakened. Goku, with his power dropping fast from the massive blast, announces that he is forfeiting the match, and then asks Gohan to take his place, because he knows that Gohan had no trouble following the fight between him and Cell. As the Z Fighters watch in awe, Goku reveals to everyone that Gohan is the only one able to beat Cell. Wanting the fight to be fair, he arrogantly gives Cell a Senzu Bean in order to recover from his energy loss, with everybody against the idea.
Gohan challenges Cell, increasing his Ki to a point that rivals even Goku's. Cell quickly launches a series of attacks on the young Super Saiyan, but Gohan is surprisingly fast. Cell eventually launches him in a mountain. Believing that he has finished Gohan when he had launched him through that mountain; he calls on Goku to fight him once more. Goku points Gohan out to Cell, who emerges from the destroyed mountain practically unscathed. Gohan asks Cell to end the killing and tells him about his seemingly limitless dormant power that reveals itself whenever he goes berserk from rage. Cell is amused by this statement and decides to test it, attacking Gohan ruthlessly. When Cell fails to get Gohan angry, he then decides to provoke Gohan by attacking the Z warriors, and quickly swipes the Senzu Beans. Goku had not taken one before, leaving him weak and vulnerable to Cell. Android 16 launches a surprise attack on Cell, grabbing him and attempting self destruction. Krillin informs Android 16 that his self destruction device was removed by Bulma when she and her father repaired him. Cell escapes from 16's grasp and proceeds to obliterate him with a single blast.
Cell then returns his attention to Gohan's friends and, instead of wasting energy and risking defeat by fighting them directly, he creates seven miniature versions of himself, which he calls the Cell Jr.'s. The Cell Jr.s, each nearly as strong as Cell himself, attack the Z Fighters, effortlessly beating all but Vegeta, Trunks and Piccolo. At this point, it is revealed that even though 16 was destroyed by Cell, his head was still operational and happened to land where Mr. Satan and his pupils were hiding. Satan surprisingly proves his worth by tossing 16's head over to Gohan. 16 reassures Gohan that sometimes words cannot reach people like Cell, and tells him to release his power to protect the Earth. Cell then crushes the head of Android 16. Finally pushed beyond his emotional limit, Gohan's inner power awakens, transforming him into a Super Saiyan 2.
Gohan snatches the stolen Senzu Beans from Cell, who could not see how Gohan had managed to do so. Gohan then effortlessly annihilates the seven Cell Jr's. It is then revealed that Goku was aware of Gohan's superior abilities all along, and Piccolo realizes that Goku used his own fight with Cell to give Gohan a preview of Cell's abilities; he never intended to beat Cell himself.
With the Z fighters out of harm's way, Gohan returns the Senzu Beans and then utterly dominates the evil android, severely injuring him with two simple hits, even after Cell increases his Ki to maximum. All the Z Fighters (except Goku) are in awe of Gohan's awesome power, especially Vegeta, who has had his pride hurt even more (it took him his entire life to become that strong while it only took Gohan a couple of years). Attempting to make sense of this child's power that far exceeds his own, Cell fires a massive Kamehameha, knowing that if Gohan dodges it, it would destroy the Earth; to his surprise, Gohan easily counters it with a larger and stronger Kamehameha, pushing the blast back and destroying a significant portion of Cell's body. Goku tells Gohan to finish Cell, but Gohan, still in a rage from his awakened power leaves Cell to suffer. As Goku predicted, Cell regenerates, and out of frustration and rage, transforms into a state that maximizes power, just like the USSJ. Trunks makes note that it was the same mistake that Cell mocked him for when he had fought him as a USSJ. Gohan beats Cell so hard that he is forced to regurgitate Android 18, which reverts him back to his Semi-Perfect form. Left with no other options, Cell initiates his self-destruction sequence, pumping his body up with enough Ki to destroy the Earth. Gohan is unable to do anything, and Goku steps in. He teleports Cell to King Kai's planet where Cell explodes, killing all life on the planet, including Goku.
After regeneration, Super Perfect Cell uses Instant Transmission to return to Earth just moments after his seeming death, announcing his presence by killing Trunks with a single shot. Vegeta, shocked by the death of his son, launches a furious assault on Cell; it is in vain, though, as Cell is unaffected by the attack and quickly slams Vegeta aside. Cell then attempts to finish Vegeta off with a second ki blast, but Gohan intervenes and takes the full force of it himself, as a result, breaking his left arm. Cell wastes no time after this and fires a massive Kamehameha at Gohan, hoping to destroy Gohan along with the planet. Gohan, with spiritual assistance from Goku, launches a counter Kamehameha with his right arm. The two blasts meet in the middle, and a power struggle between Gohan and Cell ensues. Cell has the upper hand when the Z Fighters jump in. Their attacks don't affect Cell, though, and he maintains his blast. The only attack that affects Cell at all is a powerful blast from Vegeta which throws Cell off, and allows Gohan to destroy him.