Joker frees Luthor with a mother box and forces him to spend the day together, hopping all over the world, with the Justice League hot on their trail trying to catch up.
The Flash is making his nightly visit to a high-security prison and runs around at super-speed checking the prisoners' cells. As he talks to one of the guards, the Joker (dressed as though he's been on holiday in Hawaii) suddenly appears out of thin air and greets Flash before disappearing again through a travel portal. Flash instructs the guard to lock down the prison. Elsewhere, Lex Luthor is in his cell - complaining innocently that he had only built a bomb of ultimate destruction and had not used it - when Joker appears and disposes of the guard. He enters the cell using a travel portal, and Luthor can see that he is holding a motherbox. Joker tells him that he has obtained it from Desaad (one of Darkseid's Elite from Apokolips) and, as Flash appears, he vanishes taking Luthor with him.
They rematerialise knee deep in water near a beach and Joker tells Luthor that he intends to treat him to the best day ever to allow him to get over his debacle with his bomb. Luthor does not believe him but cannot get a straight answer. Joker warns him against going ashore and, when he does, is confronted by an irate Wonder Woman with sword and shield - they are on Paradise Island. Joker grabs him before the superheroine can apprehend him and they disappear, travelling to a theme park with a high water slide.
In the Watchtower, the Justice League (Batman, Superman, Wonder Woman and Flash) observe them remotely but only see them harmlessly enjoying themselves. Batman is determined to discover Joker's true motives and magnifies his to-do list to see their next stop as the North Pole. The League move to the Fortress of Solitude but only find it daubed with grafitti as Krypto the Super Dog has chased off the villainous pair. Returning to the Watchtower, they find that Joker has been there, but this time Ace the Bat-Hound has seen him off. Batman warns other League members to be vigilant, so when Joker, less the seat of his pants, visits a casino, Zatanna is there but can't detain him. At a racing car circuit, Cyborg has no better luck, and Green Arrow's arrows can't stop him at an art gallery. Only at the Pyramids, where Joker uses the motherbox to shoot at Hawkman, does he ask Luthor where to go next.
The latter decides to seek refuge in his secret hideout but when there, as he tells Joker about the hidden bomb of ultimate destruction, he hears the motherbox charge up as though to shoot him and realises what the Joker's plan is. The latter instructs him to reveal the bomb and then starts the countdown. Luthor now sees that the many locations visited have attracted many superheroes in pursuit so that they will be destroyed in the blast. Duly, all the previously mentioned heroes now arrive and a battle begins and yet they are unable to get the better of Joker and his motherbox and begin to fall. Luthor dons his battle suit and takes out the rest but, as the countdown ticks down, he changes his mind. Tricking the Joker, he knocks him out and stops the bomb.
Luthor is returned to prison where he expects to get some peace and quiet but finds that he now has a cellmate: the Joker.
Appearing in “Best Day Ever”
"I'm obeying the rules, WW! No men's foot on the island."
- The Joker to Wonder Woman.
"And by best day you mean last day."Lex Luthor to The Joker.
- Big Belly Burger, the fast food joint Joker stops at, is a chain of fast food restaurants made famous in the Arrow Multiverse.
- Joker vandalizing the paintings in a museum is a reference from the 1989 "Batman" movie.