Disclaimer: I do not own DBZ.
Warning: Yaoi.
Sooner rather than later, it becomes clear to Bulma that Trunks will have to go back.
Trunks realizes it, too. She knows to the minute when exactly the inevitability occurred to him, because that was the moment he stopped looking like something she'd found rolled up in some burlap under a ruined city block. He trains for hours every day in the ki-dampening field Bulma developed nearly ten years ago, his face bright in a way she's never seen before. He's always been such a serious boy, premature lines of worry and anger etched into his face by all the losses he shouldn't have to had to endure, the responsibilities he shouldn't have had to shoulder. Seventeen years old and the only protector the world has left, the last line of defense.
It isn't fair, and Bulma knows it.
But it hurts her heart to see her grave little boy in love with a dead man.
Because that's what Son Goku is: dead. No ifs, ands, or buts about it—Goku died nearly twenty years ago, unconscious and in agony. Bulma was there. She saw his last breath, his final heartbeat, the flatlining of his pulse monitor. She was the one who helped Chichi arrange the burial, she was there to see her best and oldest friend go into the ground. Son Goku died, and the world died with him.
How could she not have seen this coming?
She'd been so preoccupied with thoughts of Vegeta. She's spent so many nights awake, worrying about what it would be like for Trunks to finally meet the father he'd never known. Vegeta had always been so—Vegeta. Not even twenty years of death and destruction has done anything to soften Bulma's memories of that infuriating Saiyajin. But at least she is an adult, she'd actually known Vegeta, she's had time to reconcile herself to losing whatever the hell it was that they'd had, if it had even been anything. She made peace with the prince's memory a long time back.
But Trunks—her baby, her little boy. He's always had so many questions about his father, what he was like. Longing to know if he was the sort of son his father would have wanted—and Bulma had never had the heart to answer honestly. One of the last things Vegeta ever said to her was that he had no use for baseborn half-breed brats. The bastard had always known how to turn a phrase. Bulma had known—knows—that Vegeta hadn't really meant it, that that had just been him being the Prince of All Saiyajins (like that was worthy anything now), but even so, she's determined to never tell Trunks. She's always thought he wouldn't have been able to understand.
So it had never occurred to Bulma that the danger to her baby wasn't Vegeta.
Which makes her feel stupid. Stupid as all hell. Because she remembers Goku, she remembers him and remembers what it was like to be around him. She remembers what it was like, how to know Goku was to be a little in love with him. He was like a black hole, or a supernova—his influence inescapable. She had talked about it once with Gohan, one night when Trunks was a baby and fast asleep, in one of those brief silences between rampages, and the thing they had both agreed on was how impossible it had been not to love Goku, to want to be around him, to—to some extent—even want him, at least a little.
Mom was crazy about him, Gohan had said. That's why she put up with so much. She told me once that she was lucky to have gotten to him first, because of all the people who wanted him later. She said she knew Dad would always come home to her, because she was first. Then, Gohan's voice had lowered. She used to be really jealous of Piccolo, you know? And...and...
Gohan had blushed, which had surprised Bulma then because it had seemed to her that Gohan had been too careworn to blush for many years. And maybe anyone else would have thought that Gohan had been about to say and she was jealous of you, but Bulma had always been a little too in touch with reality to lie to herself like that.
And Vegeta, she'd finished for him. Oh, don't look like that, Gohan. I think pretty much the only two people who didn't know Vegeta wanted to get his teeth into Goku were Vegeta and Goku.
Gohan had bitten his lip. But of course Vegeta would never admit it. Just like Piccolo. And—and do you remember when Turles was here? For Dende's sake! Do you have any idea what it's like for your dad to be the galactic DILF?
Ha! Oh! Oh my gosh, shhh, Gohan, don't make me laugh, I just got Trunks to sleep—
So Bulma remembers what Goku was like, his face, his eyes, his smile. She knows what it's like to stand next to him and feel completely, totally safe, like nothing could ever touch you. She knew Son Goku when he was a boy and she knew him as a man and then she knew what it was like to watch him die and feel as though all hope was gone.
She'd barely managed to survive Son Goku, and now she's watching her baby walk blindly into that same fire.
The scientist in Bulma tells her that she has to stop this now. Now, before it's too late, says Dr. Briefs. Now, while it's still only a boy's first love. Extinguish it now, while you still have some influence over your son. Hurt him if you have to, it won't hurt nearly as much now as it does later. Even if he hates you for it, save him now.
But the part of her that is a mother...the part of her that knows Trunks, that watched him grow up and still worries over whether he's eating enough or having nightmares, the part of her who knows even before he does how he's going to think things through and what conclusions he'll reach...
This part of her is selfish, and all she can think about is how she wants her baby to just be happy for once, to have something he wants, no matter what the cost is to anything or anyone else.
And on the day Trunks comes to her, his expression shy and guarded and defiant all at the same time, and shows her the blueprints he's drawn up of a standalone time-traveling device based on a smaller version of the probability engine she developed, a device which will not require someone to sit in it but merely be somehow in contact with it—
When he tells her, in a voice that doesn't waver, "I want something like this for the next trip."
That selfish, cruel part of Bulma, the part of her that is a mother looking at her only son, says, "All right."