Still feeling guilty over Fern's death, Princess Bubblegum tasks Finn to protect the banana guard from a monster later dubbed "the Grumbo". Finn can't bring himself to slay the monster as he keeps seeing Fern's face materialize when he's close to defeating it. Later, Finn meets up with Huntress Wizard (voiced by Jenny Slate) who is also trying to slay the Grumbo and she tries to give encouraging words to Finn. They find the Grumbo's cave and Finn once again stalls. Huntress Wizard puts herself in harm's way and Finn imagines Fern in her place and finally slays the Grumbo. They realize that the Grumbo was artificially created and the mysterious man, revealed to be Uncle Gumbald, plots to create another monster.

Cartoon Network had all 4 new episodes as one, couldn't find just this one by itself