One very cold winter's day, Toby is puffing through the snow. By the farm, he sees the farmer waving a red flag. The farmer explains that his sheep are lambing and he needs a vet urgently, but all the roads are blocked. Toby promises to help and races to the next station where he asks the stationmaster to telephone the vet. The stationmaster agrees and also informs the Fat Controller.
When the Fat Controller arrives at the station, he calls Toby a really useful engine, but decides to send the vet to the farm on Duck. Toby is disappointed, but understands that, due to his snowplough, Duck is more suitable for the job. Toby wishes Duck good luck as the green engine starts off through the snow. It is not long before Duck comes back; the snow is too thick and he cannot reach the farm. Toby volunteers to have a try. The Fat Controller agrees, but warns Toby to be careful. The vet boards Toby and the tram engine sets off.
Toby and Henrietta slowly climb the snowy hill and start to cross a bridge over a frozen stream. The driver is anxious, but Toby battles bravely on until he reaches the farm. As soon as Toby stops, the vet runs into the farm. After a while, the vet speaks to Toby and his driver. He tells them that all of the lambs are well, but they need somewhere to get out of the snow. The vet then asks if the lambs can sit inside Henrietta. Toby and Henrietta are happy to help and they puff gently up and down the line, rocking the lambs to sleep.
In the morning, the snow has melted and it is time for Toby and Henrietta to go home. The farmer thanks Toby and Henrietta and tells them to come back soon. The sheep bleat their goodbyes as Toby and Henrietta puff back down the hill.