The role of Theseus in book 7 of Metamorphoses is a kind of anti-Medea.
Whereas Medea is presented to us as the very embodiment of evil, Theseus, the great warrior-hero, epitomizes all that is good. It's no surprise, then, that Medea tries to kill Theseus.
She does this by preparing him a poisonous mixture containing some deadly wolfsbane that she'd brought over from Scythia. According to legend, the poison originates from the dripping fangs of Cerberus, the multi-headed dog that guards the gates of the Underworld.
Thankfully, Medea doesn't succeed in her dastardly plan, after Theseus's father, Aegeus, knocks the poisoned drink out of his hand. Medea then makes herself scarce and vanishes into thin air.
So grateful are the good folk of Athens for Theseus's return and for his surviving such a wicked attempt on his life that they celebrate at a huge feast thrown in his honor. Theseus's evident popularity is just the thing one would expect in a hero, someone who's the exact opposite of the wicked Medea.