Sam Smith is a highly-skilled steam engine driver who can operate the diesel variety as well. One day, upon returning to the depot, Sam receives a notice instructing him to report to the District Electric Department to take a learning course.

Sam is to learn how to operate a new machine - one with electric traction. The training looks complicated, but Sam is a natural. The instructor announces he has passed with ease, and Sam tips his cap in acknowledgement.

Now it's back to his old job - steam some days, diesel others. Then one day Sam arrives to work to see a shiny new electric engine - he is to be its driver. Sam climbs aboard and rubs his hands with excitement. He remembers his lessons well and Sam's electrics trains run 'smooth as silk'.

The weeks pass happily until one day Sam encounters a problem. Driving a non-stop express, miles from any station, Sam is shocked to find that the little light that tells him everything is going ok has gone out. His train loses speed until it has come to a complete stop. What will Sam do now? How will he get the passengers to their destination by four o'clock?

Inside each person, the narrator confides, there are different sides - some better, some worse. Sam calls upon the first side of his personality - First Sam. First Sam is stubborn and selfish and does little good. He leans on the signal post, thinking of the pints he is surely missing at the nearby station. He jeers at the businessmen passengers and leers at the ladies.

This side won't do at all, thinks Sam, who then summons up Second Sam. This fellow may be 'a pretty good guy' but his tendency to become 'overanxious' results in Sam hyperactively checking each and every part of the train, dismantling engine parts and wreaking general havoc.

There is only one Sam left, and that is Third Sam. This Sam thinks through the problem conscientiously. After consulting two separate dials, he concludes the problem must lie with a bad fuse. Through careful deduction, he locates the faulty fuse and replaces it. He then moves back to the driver's seat - hoping for the best - and the little light comes back on! The train begins to move, and - having sorted his characters out - Sam is back on track.