It's a Plan(et)

Midoriya Izuku used to laugh when asked that thought-provoking icebreaker question in middle school: Would you rather be stranded on an island alone or with your worst enemy? Kacchan would demolish a deserted island, make it his bitch; it would be a paradise by the end of the week, Izuku knew. Of course, he'd want Kacchan there, even if it meant Kacchan taking out his misplaced anger at being stranded in the first place out on him. He'd reap the benefits in the end, he knew. And with no one around to be an audience, to interfere, their friendship might even rekindle.

Oh, if only Kacchan was really his "worst enemy." That was silly middle school thinking, Izuku now knew, as a third year student at UA.

What's worse than being abducted by aliens? For Izuku, it was being abducted by aliens with his long time nemesis, Shigaraki Tomura. When Izuku is lucky contestant number one to be chosen to be experimented on, he makes it his mission to make sure that Shigaraki does not meet the same fate. It's not super surprising when Shigaraki isn't grateful, but hey, it's a plan, and Izuku's sticking to it, determined to be heroic whether Shigaraki wants him to or not, whether Shigaraki trusts him to be or not.