Backstory for both: In the early days of the rebuilding of Cybertron, many Cybertronians wanted to see if sparklings developed inside a mech or femme would be stronger/healthier than those left to develop out in the open. Somehow, Knockout found himself, begrudgingly, in this program. He carried a sparkling full term, thus creating Sideswipe. For a long time, Sideswipe didn't understand why people didn't trust his carrier, but then learns about it in school and kinda gives him the cold shoulder.
"You don't have any right to tell me what I'm allowed to do!"
"Sideswipe, I am your carrier."
"Yeah, but you also used to be a Decepticon!"
"You've known that." .... "But... now you know what that means."
1) Due to the concern of some Autobots on Cybertron, Knockout ends up on Earth to be the new medic for Team Bee, though unaware that he'll also be tending to Sideswipe. When he arrives, Sideswipe pretends not to recognize him, so he does the same. Eventually, Sideswipe somehow gets infected with cosmic rust. He's quarantined, but Knockout is determined to save him. So he goes in without any sort of protection, since there's no time to get any, and works frantically until he's able to save Sideswipe. Problem: Now, he's got cosmic rust. Sideswipe gets really upset because he realizes that he mattered the most to Knockout, even above his own life, and finally comes clean to the others about Knockout being his carrier. They contact Cybertron and manage to get Ratchet to Earth to save him, this time with protective gear to prevent further contamination. It's almost too late, to the point where Knockout's spark, as a defense mechanism to try to keep infection from spreading further, releases a lot of its energy. Enough to make it seem like the spark signature has gone offline. He stays 'offline' for a few days, and the others have kept his frame in good condition for a funeral of sorts that they were going to have. Sideswipe spends a lot of his time by where they've stored his frame.
-Ratchet has had Knockout on a spark monitor that can be heard throughout the entire scrapyard and Sideswipe is trying to convince himself that Knockout will survive when he 'flatlines'
-Sideswipe has to awkwardly explain to the humans that, in their terms, Knockout is more like his 'mother' than his 'father'
-(unrelated to most of the plot) Knockout convinces Bee to look up Raf and visit him (50 years after Prime? I think I read that in a fic, but I don't know if it's canon or just a number. Either way, I'm good with that number)
2) Some sort of technical accident sends Sideswipe into the past... into the days of TFP. He fanboys a little bit when meeting Team Prime, but quickly hides it. He lies about where he came from for now, joining them in their fight against the Decepticons. However, he has to tell Optimus the truth after a battle that both he and Knockout took part in. ("I mean, yeah, he makes me mad a lot, but I can't fight my carrier...") Optimus then pulls him from fights with Knockout/strategically places him on the other side of the battlefield. He has to tell Ratchet about Sideswipe being from the future, though he leaves out his relation with Knockout, in order for them to find Sideswipe a way home.
-Sideswipe gets captured by the Decepticons and 'interrogated' by Knockout
-Knockout makes some comment of how 'Sideswipe kinda reminds him of himself'
-The kids, mostly Miko, notice Optimus keeping Sideswipe away from Knockout and try to come up with all sorts of crazy reasons for it
-(going with the above statement) "Maybe he's from the future and Knockout's his dad!" ..... "Nah, that's not right."