The Question and Answer section for The Lightning Thief is a great resource to ask questions, find answers, and discuss the novel.
They meet the God Ares in a diner. He says he wants Percy and his friends to do him a favor. He left his shield at an abandoned water park nearby while on a date, and he wants them to retrieve it. Percy says they already have a quest, but Ares...
Apparently Grover has messed things up in the past.
Zeus thinks that since Poseidon and him are fighting Poseidon might have gotten his son to steal the bolt.