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Henry and Hiro have picked up a heavy load from the quarry when Hiro notices something rather odd; the smoke coming from his funnel is really dark. Henry notices that his smoke is dark, too. Henry also hears a strange clinking noise. Then, Hiro sees Duck coming towards them on the opposite track. Duck tells the two engines that some of the locomotives on the railway have been given bad coal. Duck's job is to get some fresh coal to replace it.
Henry wants to wait until Duck returns with the new coal, but Hiro disagrees and insists that they keep going. Henry is too worried and goes back to the shed, leaving Hiro to carry on without him. Hiro trundles to the docks with half of the heavy train and then goes back to get the other half. All the time the smoke from his funnel gets darker and darker and there is a terrible noise coming from his firebox.
The Fat Controller asks Hiro to collect a load of iron girders from the Smelter's Yard. Hiro really needs his firebox cleaned out, but he doesn't want to say no to the Fat Controller. Now, he really needs Henry's help.
At the sheds, Henry is amazed that Hiro had been able to take all the heavy trucks on his own. He realises that maybe he can keep on puffing, too. So the two engines set off to collect the girders. Suddenly, Hiro grinds to a halt; his firebox has completely broken. This time, Henry doesn't stop puffing and carries on, pushing Hiro and all the flatbeds all the way to the docks. The dock workers all cheer when they see what Henry is doing.
The Fat Controller is delighted. Henry says that his courage is all down to Hiro. And with that, the two engines set off to get their fireboxes cleaned and filled with new coal.