- "I'm more than a man. I'm an idea. A philosophy. And I will live on in the shadows within Gotham's discontent. You'll be seeing me soon. Au revoir."
- —Jerome Valeska's last words to Jim Gordon, moments before letting himself fall to his second and final death.
The Arkham inmates are still running freely in Gotham and they're only getting more difficult to catch. Gordon develops a plan, but reluctantly has to turn to Bruce for help. Meanwhile, Barbara finds out just how far her new friends are willing to go for her.
Jerome Valeska kidnaps a group of Gotham City dignitaries including Mayor Holden Pritchard and Police Commissioner Reynolds. Taking over a musical festival, Jerome threatens to kill the hostages unless Bruce Wayne and Jeremiah Valeska are brought to him. The festival takeover is a distraction; the rest of the Legion of Horribles are busy commandeering a blimp to dump laughing gas over a large number of citizens. Oswald Cobblepot is not totally onboard with the plan; the Legion knock him out and put him on the blimp as well.
Bruce and Jeremiah agree to help the Gotham City Police Department. Bruce is given a signal jammer to stop Jerome's bomb detonator, and the GCPD manage to save the mayor. Cobblepot wakes up in the blimp and finds the pilot hypnotized by Jervis Tetch. He manages to contact Jim Gordon and works with him to subdue the pilot and then guide the blimp out over the river. Gordon chases Jerome onto the rooftops and shoots him, knocking him off the roof. Jerome clings to the roof and Gordon tries to pull him up but Jerome declares that he will live on forever before willingly falling to his death.
Barbara Kean continues to learn about her destiny as the leader of the League of Shadows. It leads to a rift between her and Tabitha Galavan who believes she is delusional. Tabitha is then kidnapped by another faction of the League who are still loyal to Ra's al Ghul.
Arriving home, Jeremiah finds a final gift from Jerome – a jack-in-the-box filled with laughing gas.
- Bruce celebrates a birthday in this episode, and while no specific age is given, the birthday cake seen has eighteen candles on it, thus making Bruce 18. 
- At the beginning of the episode, a large painting can be seen behind Holden Pritchard during the meeting at the dining room. This is a replica of a real painting called The Officers of the St Adrian Militia Company in 1633 by Dutch painter Frans Hals.
- This episode heavily implies Jeremiah Valeska's dark turn toward a role resembling The Joker.
- The title references the song of the same name that was taken from 1953 musical The Band Wagon.
- During the introduction to the episode, Jerome asks to the mayor a popular quote made by Heath Ledger's Joker in The Dark Knight: "Why so serious?"
- The song that the Jerome's band plays duirng the GCPD arrival is the main theme of the Batman TV series (1966-1968) starring Adam West.
- Jeremiah Valeska meets Bruce Wayne and Lucius Fox.