Ivor the Engine hears the sound of a Welsh choir for the first time, and wants to join in, though he knows this is impossible.
Jones the Steam is concerned that Ivor is not himself. Along with Dai Station, they visit the Chief Engineer, who tells them that Ivor is physically in excellent condition, but that obviously something is bothering him, but what?
Dai the Station has the idea of taking Ivor back to where he first became sad. On the hill above the choir practice, Jones the Steam discovers what is bothering Ivor.
Evans the Song agrees to listen to Ivor's whistle, but it is too weak and rough for the choir. Jones the Steam determines to change Ivor's whistle for proper pipes, capable of making a more pleasant sound.
With the help of all of Ivor's friends, Jones is able to replace Ivor's whistle with three organ pipes donated from the local roundabout. Now all they need is the choir committee and the railway management to agree to Ivor joining the choir.
Evans the Song convinces both the choir committee and the railway bosses, and Ivor joins the choir.