Nils and Gorgo, on guard duty, find a bag full of biscuits but a lemming appears and steals it. Trying to get it back, Gorgo flies into a rock and Nils falls off his back into a crevasse with the rodent. Now they are both trapped and will have to learn to communicate if they want to escape. But this is not easy as the lemming only makes squeaking noises. As to Gorgo, he has gone back to the flock's camp but cannot remember what happened to Nils. Akka and Martin try to help him to recover his memory so that they can find the boy. Meanwhile Nils discovers that there is not one lemming but a whole colony of them who fell into the crevasse and who are now all stuck there. Nils finally manages to create a bond of friendship between the geese who came to rescue him and the lemmings; now everyone can leave the crevasse.