When Theseus started becoming of age, he desired to marry with a daughter of Zeus before he died. His choice fell on Helen of Sparta who, although still very young, was of exceptional beauty.
So Theseus, together with Pirithous, the king of Larissa, went out in Sparta and kidnapped Helen. Then they brought her to Aphidnae, a small city outside of Athens, to be taken care of Aethra, the mother of Theseus.
After that, it was Pirithous' turn to choose a wife and he opted for Persephone, the goddess of the Underworld. So the two friends descended to Hades in order to fetch her. But Hades understood that the intention of his visitors was to capture Persephone, so he threw Theseus into prison and let his dog Cerberus tear Pirithous to pieces.
By the time Theseus was in the prison of the Underworld, Helen's brothers Castor and Pollux, also known as Dioscuri (dios kouroi), meaning the "Sons of Zeus" , invaded Aphidnae, rescued Helen and brought her back to Sparta.