Finn got thirsty so he drank the forgetfulness water that the Bones guy handed him. That gave the skeleton dude enough time to snatch up Jake and take him prisoner. Princess Bubblegum is going to help Finn on his quest to save Jake by combining objects and potions together to make them stronger with the power of science and alchemy. Make sure you push the button to grab your items every time or you will lose them. When Finn is fighting you basically have no control other than dropping potions into the potion slot to help him out.