Cerberus, the three headed dog that guards the under world is loose. And the kids guess that it was Cronus who set him free.
So now he's causing chaos in the city and they learn from Chiron that there are only two ways to calm him: cake and music. Music works best but it can't be any music. It has to be the music from Orpheus' lyre. The only problem is that none of them could play such an outdated dorky instrument. Um, actually Theresa can.
So the Titans split into teams. Jay, Archie and Theresa go down to the underworld to retrieve Orpheus' lyre and the others get to stay and distract or preferably catch Cerberus. Joy for them. First they try luring Cerberus into a trap set by Odie with cake, which leads to...success! At least until Cerberus digs his way out. Back to the drawing board.
The other three pay the boatman to take them to the gates of the underworld. As they go to walk through only Theresa makes it before the gates shut fast. Here's Cronus and he's come for the same reason they have. And he promises Theresa that he won't kill her friends if she brings the lyre to him. After a short hesitation Jay tells Theresa to go, they'll be fine.
While they wait for Theresa's return they learn that yes it was Cronus who set Cerberus free. But he can't control him, he needs the lyre. Thus being why he's here.
Theresa walks through a beautiful paradise, searching for Orpheus. Eventually she finds him lying on his side playing his lyre. As she approaches him he mistakes her for his dead wife Eurydice and begins to call her his beloved. It's with this false identity that she gains the lyre from him. And off she goes back to the gates, little does she know that Orpheus' is not gone for good.
As she reaches the gates Cronus is ready to snatch the lyre from Theresa. The three attempt to keep up a fight but not to much prevail. Then Orpheus rocks up with all his dead friends who cover Cronus and burn his skin. As the three go and jump into the boat Orpheus' calls out to Theresa, his beloved. Jay questions this and Theresa asks if he's jealous.
The other Titans are trying once again to capture Cerberus. Out come Herry's ropes and they manage to tie him up. But the ropes snap and Cerberus is struggling free. Then Theresa appears and plays the lyre. Cerberus slowly lulls to sleep and then Hades appears from no where. He thanks the Titans for capturing Cerberus for him. What a naughty doggy. He takes the ropes and uses them as a leash. He also offers to return Orpheus' lyre for Theresa. And so once again Titans save the day. Bravo!