Time jump:

Sumatra Road, underneath the Palace of Westminster

Sherlock and John explored the insides of the missing carriage. They shined their torches brighter as they walk deeper into the carriage. They looked at every seat, every corner, even checking the ceiling and floor.

"It's empty. There's nothing." John muttered.

Just as he said, Sherlock saw a pair of red and black cables along the wall and down to the back of one seat. He shined the torchlight at that area and lifted the cushion. He looked around. "This is the bomb."

"What?"

"It's not carrying explosives. The whole compartment is the bomb!" Sherlock said as he saw a giant explosive under the cushion. They lifted all the other cushions and there are explosives under it. As they were looking, Sherlock found a massive explosive, the heart of the bomb.

3 minutes. "We need a bomb disposal." John looked at Sherlock. Sherlock said, still staring at the device. "There may not be time for that now." 

"So what do we do?" John questioned him.

" I have no idea."

John panicked. "Well, think of something." 2:45 minutes.

"Why would you think I know what to do?"

"Because you're Sherlock Holmes. You're as clever as it gets." John said, half-sarcastically, half seriously.

"Doesn't mean I know how to defuse a giant bomb." Sherlock looked at John. "What about you?"

"I wasn't in bomb disposal. I'm a bloody doctor." John looked back at the bomb. 2:30 minutes left. "Can't... Can't we rip the timer off, or something?"

"That would set it off." 

"You see? You know things." John stated. Sherlock sighed.

As the clock continues to tick, Sherlock and John got even more panicked. "MY GOD!" John's breathing rate increased. "Why didn't you call the police?"

Sherlock tried to explain himself. "Please, just..." 

"Why do you NEVER call the police?" John, do not explode again. Sherlock sighed once more. "Well, it's no use now."

"So you can't switch the bomb off. You can't switch the bomb off and you didn't call the police." His worries turned into anger. 

"Go John." Sherlock pointed towards the exit of the carriage. "Go now."

"There's no point now, is there, because there's not enough time to get away, and if we don't do this, other people will DIE!" 2 minutes.

"Use your Mind Palace."

"How will that help?"

"You've salted away every fact under the sun!"

"Oh. So you think I've got "how to defuse a bomb" tucked in there somewhere?!" Sherlock rolled his eyes. 

"YES!!" 

"...Maybe."

Sherlock tried to go into his Mind Palace. "Think. Please think. " John reminded him continuously. "THINK!!"

John tried to keep calm but Sherlock's groaning noises are making him even more panicked by the moment. Then he finally opened his eyes. He took long and deep breaths, but then he turned to John with a sad and unsuccessful face.

"Oh my gosh." John looked around him, and the word "panic" and "worry" was written all over his face. "So this is it. This is it."

Sherlock continued to try defusing the bomb. "Turn that off. Oh, God!" He mumbled. "I'm sorry." He turned to John. 

"What?"

Sherlock's eyes were filled with tears. "I can't... I can't do it, John. I don't know how. Forgive me?"

"What?"

"Please, John, forgive me... for all the hurt that I caused you."

"No. This is a trick." John still doesn't believe his ears. "Another one of your bloody tricks."

"No."

"You're just trying to make me say something nice. It's just to make you look good even though you behaved like..." John breathed heavily. "I wanted you not to be dead."

"Yeah. Well, be careful what you wish for.

"If I hadn't come back, you wouldn't be standing there and..." Sherlock shook his head. "You'd still have a future... with Mary."

"Yeah. I know." John looked at his friend. "Look, I find it difficult. I find it difficult, this sort of stuff."

"I know."

Knowing that he has not much time to waste, he decided to forgive Sherlock. "You were the best and the wisest man... that I've ever known." John whispered. "Yes, of course I forgive you." John shut his eyes and tried to hold back tears. It's the end, he thought. It really is the end.

"Did you really think that he doesn't know how to defuse a bomb?" a voice asked. Suddenly, they both saw a young boy come out from the shadows. He was wearing a yellow t-shirt, but the colour looked a bit faded. He wore an expensive necklace, the butterfly diamond shining through the night. He looks about 10-13years old. Sherlock made a rough guess.

"Bloodly hell!" John turned to Sherlock. "It's a KID!"