Demetrius has to marry Hermia - Hermia wants to marry Lysander - Puck puts a potion into Lysander's eyes by mistake - Demetrius then returns the love back to Helena.
Demetrius watched the tale of Pyramus and Thisbe performed to him by the six craftsmen on Duke Theseus and the Duchess's wedding day at night, alongside Helena, Lysander and Hermia.