Winnie the Pooh and his friends hear a strange noise and find a set of large, perfectly circular footprints in the Hundred Acre Wood.
They jump to the conclusion that the noise and prints are from a heffalump, and Rabbit organizes an expedition to go try to catch it; but Roo is told to stay behind, as everyone believes the expedition is too dangerous for one as young as he.
Roo slips out on his own in search of the heffalump, and soon finds it -- quite by accident. "It" is a playful young four-footed creature named Heffridge Trumpler Brompet Heffalump IV (nicknamed "Lumpy"), colored lavender and having a Yorkshire-British accent and a trunk like an elephant, but being not much larger than Roo.
Roo and Lumpy strike up a sweet friendship and work together to dispel the unfounded fears of their respective friends and families