When Percy arrives in the Underworld, he must face an angry Hades.
The god of the Underworld insists that Percy stole both Zeus' thunderbolt and his helm of darkness. Percy fights and just barely makes it back to earth.
On earth, he confronts Ares, who delights in the idea of Poseidon, Zeus, and Hades at war. Ares tells Percy that he hid the bolt in Percy's backpack, which was the bolt's sheath in disguise. For his part, Percy accuses Ares of betraying him. However, Ares reveals that it wasn't actually his idea to steal the bolt and helm.
At the end of the story, we learn who was truly responsible for the theft of the bolt and helm. Luke reveals that he stole both items. Accordingly, he snuck into the throne room at Olympus and took Zeus' thunderbolt right from his chair. He also stole Hades' helm of darkness.
Luke reveals that it was Kronos who inspired him to steal both items. By all indications, Luke was easily manipulated by Kronos because of his focus on personal glory. From Poseidon, Percy learns that Zeus cut Kronos into a thousand pieces and cast the remains of Kronos into the darkest pits of Tartarus. There he stays, but his strength is growing by the day.
Meanwhile, Luke reveals that, after he stole the helm of darkness and the thunderbolt, he was confronted by Ares. For his part, Ares overpowered Luke and threatened to burn him alive. However, Kronos helped Luke make a convincing argument against it. With the help of Kronos, Luke manipulated Ares, convincing him to keep the bolt and helm hidden for a time. In doing so, Ares would foment war between Poseidon, Zeus, and Hades.
Ares, the god of war, couldn't resist the idea of a war among the gods. This is the real reason he framed Percy.