A scene of Endsville two weeks into the future shows a dystopian world ruled by the "Lord of Horror", who wields the all-powerful artifact known as "Horror's Hand". At the same time, two robotic replicas of Billy and Mandy are sent into the past, to stop the real Billy and Mandy from interfering with the current events.
In the present, Grim goes to reap General Skarr, who accidentally got a big wound in his torso. However, due to Billy and Mandy's meddling, Skarr escapes, and they make a big mess in the city. Following his failure to be an effective Reaper, Grim is taken to the Underworld Court, presided by Judge Roy Spleen, where he is sued for misuse of his powers by his nemesis, the Boogeyman. After the trial, Grim is found guilty, is stripped of his rank and magical scythe, and he, Billy, Mandy, and Irwin are sentenced to be exiled. Then, Boogey volunteers to carry on the sentence by taking the prisoners aboard his ship, and dropping them off in a river of fire; he plans to do this while he sails to obtain Horror's Hand, an object capable of transforming him into the scariest and most powerful being in existence.
The group of Billy, Mandy, Irwin, and the powerless Grim, escapes from Boogey's ship, and set themselves to reach Horror's Hand before Boogey does. They eventually meet Horror, the guardian of the Hand, at the same time Boogey and his crew arrive. Then, in order to decide who will have the honour to battle Horror for his Hand, the heroes and Boogey participate in a challenging obstacle course, which the heroes win against all odds. After dispatching Horror with ease, Billy, Mandy and Irwin reach for the Hand, but it defends itself by making their worst nightmares come to life. Then, Boogey gets ahold of Horror's Hand, but he turns out to be the least scaring creature ever, seriously injuring himself, and being abandoned by his own crew.
Then, Grim recovers Horror's Hand, and at the same time his sentence is revoked, becoming the Reaper once again. Then, the future Billy arrives, telling them Horror's Hand shouldn't fall in the wrong hands, spcifically Mandy's as he indirectly reveals that she was the one who used Horror's Hand to rule the world. When they return home, Grim seals Horror's Hand in his bottomless trunk, and the future Billy returns to his own time. In the end, the future Billy realizes the situation in his time didn't improve, and it is Fred Fredburger who obtained Horror's Hand to rule the world.
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