In Attack of the Clones, Obi-Wan Kenobi traces an assassin to a clone army on Kamino while Anakin Skywalker and Padmé Amidala fall in love, and the story ends with the first battle of the Clone Wars on Geonosis. Everything below reveals the story, so skip ahead if you have not seen it.
After Padmé is attacked a second time, Obi-Wan catches the assassin Zam Wesell, a shape-shifting bounty hunter. A poisoned dart kills her before she can name her employer. The Jedi split up from there: Anakin takes Padmé into hiding on Naboo, where the two fall in love despite Jedi rules against attachment, and Obi-Wan follows his clues to the ocean planet Kamino.
On Kamino, Obi-Wan finds a huge clone army being grown for the Republic, with the bounty hunter Jango Fett as its template. He chases Jango to Geonosis and learns that Count Dooku is gathering the separatist leaders there and building a droid army.
Meanwhile, Anakin is troubled by dreams of his mother. He travels with Padmé to Tatooine, where he finds that his mother, Shmi Skywalker, has been captured by Tusken Raiders, and she dies in his arms. In a rage, he kills the Tusken Raiders who held her. Padmé and Anakin then learn that Obi-Wan has been captured on Geonosis, and they fly there to rescue him.
The rescue fails, and Anakin, Padmé and Obi-Wan are condemned to die in an arena. A Jedi strike force led by Mace Windu arrives, a large battle breaks out, and the clone army lands to fight the droids. Windu kills Jango, and Dooku flees.
Obi-Wan and Anakin chase Dooku, who wounds both of them and cuts off Anakin's right arm. Yoda arrives and fights Dooku, who escapes with stolen plans for a superweapon.