You are cordially invited to the Wedding of the Year! But who will be united in matrimony?

Finn practices his sword fighting skills at the candy junkyard, where he meets an old gumball robot named Rattleballs. To Finn's shock, he finds out he's NOT the master swordsman he thinks he is. What the what?! Finn begs Rattleballs to train him, but he soon discovers his mentor's deep, dark secret... a secret that will ultimately test Finn's loyalty to Princess Bubblegum.