Thomas is working hard on his branchline when he hears a squeak. No one can tell where the bothersome squeak is coming from, however. So, at Elsbridge, the driver oils Thomas' wheels, but it doesn't work. Then, the guard suggests that it might be the couplings, so they uncouple Annie and Clarabel, but it doesn't make a difference. The fireman decides it must be the steam valves, so he lets off steam - but it still does not stop the squeak. The porter then suggests that it might be his new boots, but it's not the same as the squeak coming from Thomas. Next, they check the water tank. The squeak is not coming from the tank, but Thomas does need a drink. At the water column, the squeak gets louder. Later, as Thomas passes a signalbox, the signalman tells the driver that the squeak is coming from the cab. On closer investigation, the crew spots a family of mice in the corner of the cab roof. The signalman moves the mice into a hut next to the signalbox and everyone is happy, even the mice!