After Joy and Sadness get lost in Riley's mind, they meet Bing Bong who advises them to take the Train of Thought to get back to Headquarters. From then on their journey concentrates on catching the train and then getting it to go, which turns out to be harder than they expected. However, just when they finally get onboard and on route to Headquarters, the destruction of Honesty Island destroys the train's tracks, causing it to be lost in the Memory Dump.