Disclaimer: I do not own Sherlock and all rights belong to the BBC.
It wasn't a well-known secret-actually it wasn't even known to most people outside of the Holmes family knew about it. The secret was one of Mycroft's most embarrassing moments in his entire life span. Mycroft's most embarrassing moment would make you laugh and wonder how that particular moment could be embarrassing, however the most embarrassing moments for some people were the most unimportant thing could be someone's deep dark secret.
Mycroft's most embarrassing moment wasn't one of those times when you would get completely drunk and badly sing karaoke. Mycroft didn't do that -thank you very much, it was Sherlock who was the brother who was fonder of drunken singing. Instead it involved a rubber duck. The sort that you would get in a children's bath and it would happily bob along in the sea of bubbles and washed off dirt.
It was Sherlock's fault, that led to Mycroft's most embarrassing moment or what it was now known as 'The rubber duck incident' in later years. After 'The Rubber Duck Incident' it led to Mycroft's complete and utter hatred of the plastic toy. Every time, Mycroft saw a plastic duck, he had the urge to stab and beat to a million tiny pieces with his umbrella.
You may think that the incident involved something like Mycroft being forced to dress up as a rubber duck for a costume party or it was a rather stupid work name. Instead it started off with Mycroft singing in the bathtub when he thought that he was home alone. Normally Mycroft wasn't one for singing but since it was his last summer before he went to university, so he thought that he would indulge himself with a singing treat.
Sherlock was bored- well more than usual. The holidays always made his brain rot from the lack of mental stimulation as most of his experiments were deemed 'unsafe' by Mummy. Even the ones that only involved flames and no chemicals. And because of that Sherlock was bored, as there was nothing to do apart from annoying Mycroft.
After slowly walking through every room in the house and finding no sign of his brother, Sherlock was about to give up. Usually Mycroft was in his bedroom or in the kitchen, but he was gone. Sherlock thought that maybe Mycroft was robbing a bakery or doing something involving cake.
Sherlock loudly sighed as he could in hopes that Mycroft would come running to help him in ending his boredom by playing pirates together. After sighing a few times just to make sure that Mycroft heard him, Sherlock waited and tapped his foot impatiently after a few moments of waiting for his brother to drop everything for him. After a few moments, Sherlock sighed again as he realized that his brother wasn't going to help him in chasing the monster of boredom away.
"Stupid Fatcroft." Sherlock mumbled under his breath as he started to walk back to his room. "Always thinks that he is more important than me. He is not helping me stopping my boredom as he knows that I am the smarter one. He is probably drowning his sorrows in cake as he has nothing else to value."
As Sherlock made his way up the large spiraling staircase to the top of the house, he heard a someone singing, extremely badly. They sounded like an opera singer with a severe case of laryngitis. Deciding that it would be better to investigate the source of the noise as a way to procrastinate from doing nothing at all.
After minutes of fruitless searching, Sherlock was about to give up his search for the bad singer. Deciding he would finish his secret experiment on mold cultures that was in Mycroft's bathroom. After Sherlock picked up his notes on mold and made his way to his brother's bathroom. Once he was at the bathroom door, he heard the sound of the murderous singing. The song was about a rainbow, that was from that movie that his mother loved and forced him to watch. Was it from the Wizard of Ooze? Mycroft always told him that he would be the Scarecrow as according to Mycroft, Sherlock always needed a brain. Sherlock would always respond by saying that Mycroft would be the Tin man as he didn't have a heart and the only thing that kept Mycroft alive was the amount of cake his brother ate. After that comment, the two brothers ended up fighting and Mummy decided to never let them watch the movie again.
Sherlock giggled as he heard Mycroft go into the chorus. There weren't many things that Sherlock found funny, but his brother singing was now the funniest thing in this life. Sherlock quickly dashed into his room and pulled out his tape recorder that he got for his birthday and he pressed record button.
Sherlock creaked the door open slightly and saw Mycroft in the bathtub singing into a rubber duck as if it was a microphone. Didn't people usually sing in the shower, not the bathtub? Sherlock thought as he turned his face away from his brother. It felt strange for Sherlock to watch his brother singing in a rubber duck in the bathtub mostly as his brother was naked. Sherlock felt a sneeze start to build up. He tried to do all those things that books have told him to prevent a sneeze. He tried pinching his nose, that didn't work. He tried pressing his tongue against the roof of his mouth, that didn't work. He tried to think of something obscure, that didn't work mostly as he couldn't think of something that obscure.
"Somewhere oovvvver the rainboooooooooooow." Mycroft sang loudly in a high voice that somewhat resembled an opera singer who had been kicked into the groin. Just as Mycroft finished the long note, Sherlock had sneezed. It wasn't a quiet little one that he usually had, it was the opposite. It was a loud sneeze that was actually one that consisted of a few big sneezes. After the first loud sneeze he stopped signing. Mycroft noticed the gap in the door, he was sure that he shut the door completely. Another sneeze, he noticed the noises from the sneezes were close. Mycroft squinted slightly and he managed to get a glimpse of his brother holding a tape recorder. Mycroft grabbed his towel and left the bathroom with the rubber duck in his hand. "You little brat!" Mycroft shouted "Give me the tape recorder." Sherlock noticed that his brother was slightly angry at him and started to run away from his brother who was wet and naked apart from the towel he was holding at his waist with a clenched fist.
Sherlock ran down the stairs with Mycroft not far behind him. Despite being quite chubby Mycroft was a surprisingly fast runner, Sherlock reckoned if Mycroft wanted he could join an athletics team and loose that extra weight, but it would never happen as Mycroft was rather fond of Mummy's cakes. Mycroft did eat most of Cousin Matilda's wedding cake and he blamed it on Uncle Vincent's dog. Sherlock heard voices from the parlor, the voices were quite nasally and squeaky at the same time if that was humanly possible. It must have been Mummy and her friends in their monthly 'get together' where they would drink wine and complain about clothes and the stock market. Every time they would meet, they would have to have a picture to capture the memory of them being together as they didn't see each other for weeks, and they need the remainder of what each other looked like and who was at each gathering each month. The women were at every gathering, no one new entered or left, apart from Miss Plimpton who had drank too much champagne and tried to get into bed with Mycroft , but that was another story.
Sherlock felt something hit off the back off his head and it went flying forward. Sherlock looked at the ground and saw the rubber duck which was lying by the large cake Mummy insisted to have at every get together. Mycroft had thrown a rubber duck at him, it was actually the most normal thing Mycroft had thrown at him. In the past Mycroft had thrown: Videos, microscopes, shoes, an umbrella, a dead goldfish and for some reason a pineapple at him.
Mummy and her friends got ready to take a picture in front of the large cake. Sherlock ran past Mummy and brushed past her legs making her stubble in her high heels slightly. "Do be careful Sherlock, this is a new skirt I have on. I nearly went into the cake!" Mummy squealed as he friends laughed. Sherlock muttered an apology before hiding in the kitchen.
Sherlock heard a large thud and some screams from the living room. Sherlock knew that he couldn't be the cause of them, mostly as he only touched Mummy as he ran past. He peeked his head through the door and found that the cake was all over the floor and Mummy and her friends had cake in their hair. Mycroft was covered in cake and was collapsed on the tray where the cake was. Mycroft quickly stood up and apologized and then Mummy screamed and pointed at Mycroft. Mycroft was curious why his mother was screaming and pointing at him.
"Mummy, I am not that fat and pointing and screaming at me won't help my self-esteem." He commented before looking down and realizing that his towel was on the other side of the room. Mycroft quickly tried to hide himself from everyone else, but mostly it was so he could keep the little amount remaining dignity he had left. A bright light and a clicking sound brought his attention. He noticed that Sherlock was behind the camera and was cackling like an old hag.
The door opened and father came in from work. Father took a look around the room and assessed the situation. It was dreadful and he had seen cleaner stag nights compared to the mess his living room was in. Sherlock noticed his Father's turn bright red; it was something that happened before he shouted. With an almighty roar he shouted "Mycroft Holmes, put on some trousers on at once!" Mycroft grabbed his towel and ran upstairs wondering why fate had made him related to his brother as he was the one who caused all this and Mycroft was just the victim as always.
After that incident Mycroft had developed a complete and utter hated of rubber ducks. After managing to get all the cake off himself in a song free bath, Mycroft vowed that he would never sing again and he would murder his brother and also to destroy all rubber ducks, in that order.
Every now and then, depending on how much of a twat his brother is being, Mycroft always expects a phone call from Sherlock. The phone calls always consist of Sherlock quacking down the phone and the old recording of Mycroft singing a song about the rainbow.
Also every year at Christmas and other gift giving occasions, Mycroft always gets a rubber duck from his brother. Mycroft has tried to throw away the rubber ducks from his brother, but every time one gets thrown away, ten more get placed his bathroom somehow. At the moment Mycroft has a rather large collection of rubber ducks that almost rivals his umbrella collection.
And that is the deep dark secret of Mycroft Holmes. It may not be the most amazing secret or the most gossip worthy, but it is enough to destroy Mycroft's political career and that is why Mycroft likes to pretend it didn't happen. It is not that bad that Sherlock can mock him on his hatred of rubber ducks as Mycroft knows some embarrassing stories about Sherlock, such as the reason why his brother has a pair of pink skinny jeans in his wardrobe.