It was a sunny day on Sodor, and all seemed well as Percy chuffed through the countryside. Everything was running to plan on the railway, and Percy was adamant that nothing would change that.

As he pulled into Knapford, he became aware of a slight commotion. Thomas appeared to be stifling laughter at Edward, who was covered in what looked to be half a tin of white paint. There was a rather sheepish looking porter on the platform, whose trolley was overturned in such a way that most of its contents were strewn across the platform. And Edward.

As Edward regarded Thomas with a long-suffering look, the tank engine could only choke out a muffled "F". Edward merely rolled his eyes, but Percy was confused,

"Thomas, what do you mean?"

Thomas raised a triangular eyebrow at his best friend, "Huh?"

"You said 'F'. What does that mean? Were you trying to swear?" Percy inquired.

"What? No, Percy. It's-" suddenly Thomas' guard blew his whistle and with a toot, he began to shift out of the station towards his branch line.

"It's what?" Percy asked eagerly.

Thomas hurriedly gave his friend a half-hearted "It's what you say when something happens."

Percy looked to Edward for confirmation, but the old engine just gave the engine equivalent of a shrug, before his own guard blew the whistle. Percy too continued on his way.

Back at the Sheds that night, Percy trundled in to a rather smug-looking Gordon – well, more smug than usual. The other engines were tittering amongst themselves, and Gordon seemed all too pleased to share his day's triumph.

"What's going on?" Percy asked as he backed off the turntable.

"Gordon's broken his first speed record in twenty-four years!" Emily replied, intentionally suppressing the knowledge that this was all the express engine would be going on about for some time to come.

Percy narrowed his eyes briefly to assess the situation, before opening his mouth, "F."

The chatter in the sheds abruptly stopped as everyone looked at the saddle tank.

"What?" he asked, confused.

"Percy," James began, face splitting into an amused grin, "you do know what that means, right?"

Percy was bemused, "Yeah, Thomas told me earlier. He said it's what you say when something happens. Gordon's speed record is of note."

"Thomas is an uncultured peasant." James sneered, earning a "hey!" from the tank engine's corner of the sheds, "'F' is what the kids say these days when something bad happens."

"Oh." Percy perked up, glad that his error had been pointed out before any other embarrassing mistakes occurred.

The engines soon settled down to sleep, and Percy was no different. A peaceful slumber descended upon the little engine, as he dreamt of taking his post train through the countryside.

The next morning, there seemed to be a sombre atmosphere about the yard. As Percy completed his daily task of shunting Gordon's express coaches into formation, he sidled up to one of the bay platforms. James, Thomas and one of the Scottish twins were also milling about the station, but soon hushed as a rather forlorn-looking Fat Controller approached them, an even more upset Dowager Hatt trailing behind him.

"What's the matter, Sir?" Thomas asked with concern.

The Fat Controller heaved a heavy sigh, "We have received the news that my aunt – mother's sister – has passed away."

The engines remained silent as Dowager Hatt dabbed at her eyes with a handkerchief. None of them knew quite what to say.

"F."

Everyone in the station, human and engine alike, looked to Percy sharply in shock. Thomas gaped. James breathed a pained "Mate…". The Fat Controller stared. Dowager Hatt started crying again.

"What did you say, Percy?" The Fat Controller asked, pure shock drowning out any anger.

"F."

The Fat Controller decided it was time to usher a positively distraught Dowager Hatt off the platform, fixing Percy with a firm glare, "I'll be having words with you later, Percy. Come now, Mother, I'm sure he didn't mean it…"

Thomas released a breath he hadn't realised he was holding, "What in God's name was that?"

James, who looked somewhere between guffawing and sobbing, replied simply, "I am not quite sure."

"What? What did I do?" Poor Percy looked near tears himself, "I was being respectful!"

His driver, who had watched the whole situation in numbed silence, merely patted the side of his cab consolingly, "Come on, Perce. We need to have a little conversation about tact."

Later that evening, Percy had been consigned to the sheds alone. An old blue car drew up on the gravel, hailing the arrival of his owner. The Fat Controller hauled himself out of the seat, and after straightening his waistcoat and top hat, approached Percy. His two bodyguards followed close by. Percy could feel the seconds of his life ticking away with every crunch of foot on gravel, and remained silent.

The Fat Controller's expression melted slightly as he gazed upon Percy's forlorn expression. He put a consoling hand upon the engine's buffers, and cleared his throat, "Now, Percy. I want you to know that I'm not angry with you. Not anymore, at least. I am of the understanding that you probably don't know quite what 'F' means. Care to explain further?"

Percy frowned, "I heard Thomas say it earlier, and he said it was something you say when something happens. Then I said it when Gordon broke his speed record, and James corrected me-"

"James. I should have know." The Fat Controller narrowed his eyes at the red engine's name.

Percy continued, "-he corrected me by saying it's something you say when something bad happens. The death of Dowager Hatt's sister was bad, right?"

"Yes, Percy. It was bad. But what James failed to explain was that it's not used in, uh, serious contexts."

"So you know what it means, Sir?"

"Yes, Percy. Everyone and their mother knows what it means."

"Oh."

The Fat Controller spent the next forty-five minutes explaining the finer nuance of the statement 'F'. By that time, James himself was returning to the sheds. Upon seeing Percy and the Fat Controller, he blanched.

"James." The Fat Controller said in a completely even tone of voice, "I hear you and Thomas may have misled Percy today, resulting in my mother becoming extremely upset."

James gave an uneasy smile, "And how is Dowager Hatt doing?"

"Not great. You shall be on shunting duty for the next two weeks. I shall think up a suitable punishment for Thomas too. Hopefully that'll give you time to think about tact." With this, he turned on his heel and headed back for his car. His bodyguards began to follow him, before one of them paused and addressed the outraged James with a solemn "F".