Percy's Lucky Day
 is the fourteenth episode of the seventeenth season.

One morning, Percy is dropping off mail at Wellsworth. As the mail is unloaded, Percy is daydreaming and mistakes a man's green handkerchief for the guard's flag. Before the men have unloaded the mail, Percy pulls away causing a sack of mail to burst open. While Percy waits for the letters to be picked up, Thomas passes by and comments about Percy have bad luck. Bill and Ben also pass and they disagree with Thomas; they think the accident is done to sheer carelessness.

Soon, Percy is hurrying along to try and make up for lost time. Suddenly, he spots some pumpkins on the line which have fall off Bill and Ben's train. Percy brakes, but cannot help ploughing into the pumpkins. Some crates fall of his train and some of the pumpkins get stuck to his wheels. He has no choice but to wait until the mess is cleared up. Presently, Hiro pulls up and Percy tells him about how bad his day has started. Hiro tells Percy that he is just having an unlucky day. As Hiro clatters away, Percy recalls what Thomas had said earlier about bad luck and wonders if he is an unlucky engine.

Meanwhile, at Maron station, the Fat Controller is talking sternly to Bill and Ben. To make sure that the rest of their load stays in tact, the Fat Controller orders that one engine go in front and the other push from behind. Bill and Ben argue about who should go where and eventually Bill goes to the back of the train.

At Ulfstead Castle, Percy is delivering the mail. He tells Stephen about his spell of bad luck and Stephen gives Percy his old lucky charm - a horseshoe. Stephen's driver hangs the horseshoe around Percy's brakepipe. Percy is very grateful and sets off, but his wheels are still sticky and he jerks as he pulls away. Unbeknownst to Percy, the horeshoe falls off.

The next day, Percy is on time and does not have any accidents. His day is so lucky that the Fat Controller gives him an important delivery; he must take the Duchess of Boxford a present. As Percy does so, he believes his lucky charm is working unaware that he does not have it.

Later that day, things are still going well for Percy until he meets up with Bill and Ben. The twin at the front of the train, Ben, has derailed and they are arguing about whose fault it is. Still feeling very lucky, Percy offers to pull Ben back onto the track. Percy tells the twins about his lucky horseshoe, but Bill is puzzled as he cannot see it. Realising it's not there, Percy races away to find it; he cannot possibly help the twins without luck on his side. Bill decides to have a go at pulling Ben back onto the track by himself, but only endangers Ben further. Percy can see the desperate situation Ben is in and knows that he must help with or without his lucky charm. Percy couples up the the twins and starts to pull. The Fat Controller arrives just in time to see Percy pull Ben back onto the rails. The Fat Controller praises Percy who rushes off to try and locate his missing lucky charm leaving the twins arguing about who was to blame for the accident.

Percy hurries back to Ulfstead Castle to tell Stephen that he has lost the lucky horseshoe. Stephen tells Percy that he never had the lucky horseshoe in the first place as it fell off as soon as he pulled away. Percy realises that the good luck he had was nothing to do with the horseshoe and he states that he does not believe in lucky charms anymore. Stephen reveals that he does not really believe in them either and had only given Percy the horseshoe to cheer him up.

A station worker also speaks. He is voiced by Keith Wickham in the UK version.