Don't know if someone else has done this, but I looked and when I couldn't find anything I decided to do this. I don't own Doctor Who and this takes place near the end of Knock Knock.

"So what happens now?" Bill asked the Doctor, while her (ex?)housemates stood nearby. "Are you going to give everyone a lift?"

"No. No I'm sure you can all find your way to your homes?" the Doctor told her politely as he continued to observe the collapsed building they had just escaped.

"Can't even pay for a taxi?" Bill asked hopefully.

"No money."

"Seriously? Then how do you pay for food and stuff?"

"How should I know? Ask Nardole, those things are part of his job."

"Hey Bill." said one of the housemates, Harry, as he walked up to them. "My gramps is on his way. I called him saying there was a problem with the house and asked if he could pick us up, if you and the Doctor need it."

"Why are you calling your grandfather? Why not your mother or father?" the Doctor suddenly butted in.

"Well me and gramps have always had a close relationship. He's a real cool guy." Harry responded with a smile, which instantly vanished when a car arrived and an angry man steps out.

"Harold! I haven't slept in a day, so this better be important." the man almost shouted as he walked up. "You're lucky I didn't call your mother, she would be murdering you for this and...is that's where you are living!?"

"Okay, I know it looks bad. But I swear it wasn't mine, or anyone else's, fault." Harry replied quickly, as he gestured around to everybody.

"He's right you know. It wasn't their fault." the Doctor said to the man, and to Bill's amazement the Doctor was smiling. And it wasn't a polite smile or a patronising one, it was a genuine smile of surprise and happiness.

"And who are you?" the man asked. "The landlord? Because if it isn't there fault, it's your fault and-"

"Oh Harry Sullivan, you're still an imbecile." the Doctor interrupted him. "Nice to see you haven't changed."

The man, Harry Sullivan, pauses and looked at the man in front of him, from the odd clothes to the ancient look in his eyes. Then he smiled himself before speaking. "So, you've regenerated again? I like it. Much better than the one in leather."

"Oh, I agree." the Doctor replied, before looking at the younger Harry. "So this is your grandson? Must get confusing."

"He's actually called Harold. He just calls himself Harry when with his friends." Harry explained.

"Hold on. Do you two know each other?" Bill asked as she and Harold observed the two old men.

"Yes. Harry here used to travel with me in the TARDIS, like you do." the Doctor explained, before looking between Bill and Harry. "Oh right, Harry meet Bill Potts. Bill, meet Harry Sullivan."

"Nice to meet you." Harry said as he held out his hand, and Bill politely shook it. "Looking after him I hope."

"Same here." she replied with a smile. "And yeah, it's kind of like a full time job."

"Excuse me?" the Doctor asked, clearly insulted.

"So, I assume something happened in there which UNIT should investigate and quarantine?" Harry asked as he nodded to the collapsed building.

"Oh nothing really out of the usual. Living woman made out of wood thanks to alien bugs." the Doctor told him.

"Can someone please explain what's happening? What's the TARDIS?" Harold asked his grandfather in confusion.

"Is he an imbecile like you were Harry?" the Doctor asked.

"Hey don't push your luck Doctor. You might be a friend, but I won't stand having you insult my grandson." Harry responded, though his eyes easily betrayed him, as it was clear he was enjoying seeing the Doctor again.

"Again, what is happening?" Harold continued to ask.

"Harold, I think I have a lot of stuff to tell you." Harry told his grandson, before looking at the Doctor. "Don't suppose you can do a favour for a old friend?"

The Doctor stared at Harry for a moment, before sighing in defeat. "Oh very well. Just make sure that Harold, and by extension his friends, don't touch anything or look at my browser history."

"Where are we going?" Shireen asked as she and the others approached.

"Everyone follow me. And when you all dash out of the doors to remind yourself how small it looks on the outside, please go in a orderly line." the Doctor told them, celalry annoyed by all of them. but that annoyance was quickly replaced with a smile when he turned to look at Harry again, and the two quickly began tot alk about old times as everyone followed him towards the TARDIS.

Tell me what you think and if there are any spelling mistakes or sentences that don't make sense. So I was just randomly looking at Doctor Who stuff when I learned about how the character of Harry was originally meant to be the grandson of Harry Sullivan after reading some old posts. After I searched and found out it was true, I came up with this small idea. By the way, to those who don't know the mention of a leather wearing incarnation of the Doctor is a reference to how Harry met the Ninth Doctor in a comic.