As told in the Aeneid, Sinon was a Greek spy during the Trojan War. When the Greeks pretend to sail away from Troy, Sinon stays behind. He allows himself to be captured. He pretends that he was abandoned by his comrades, and pleads for mercy. He then explains to them that the Greeks left the Trojan horse on the shore, as a gift to placate Athena, since they had stolen her Palladium from inside Troy.
After the horse is dragged into Troy, Sinon sneaks up to it at night and lets out the soldiers hidden inside.