Meanwhile in the original timeline...
"Anybody home?" Bulma and Vegeta had arrived to find the party had not started yet. In fact, nobody was home.
Vegeta grumbled. "Great. They invite you over, then don't prepare the food."
Something came spinning out of the sky and landed on the front lawn. It was the little turtle with Kamesennin sitting on top, looking quite green. He staggered from the turtle's back and threw up in the nearby bushes.
After he had finished emptying his stomach, the old kungfu master of the Kame School of martial arts turned to the little turtle. "Thank you, little turtle. You don't have to pick me up later. I'll go home with Marron and #17." After the turtle left, Kamesennin grumbled to himself about having to ride a turtle, having no transport since Kuririn and #18 had taken their plane on their honeymoon and the other was used by Marron and #17 to come here. Then he caught sight of Bulma and Vegeta.
"Oh, hi Bulma." The woman was still lovely, even after all these years. How he'd love to touch her, but of course he didn't dare. Not with Vegeta around. "Hi Vegeta. So where's the party? I brought some sweets, but I accidentally threw up on them, so I threw them away."
A car came sppeding and halted abruptly inches away from the front door. ChiChi came out carrying loads of foodstuff. "Oh, hi Bulma, Vegeta, Kamesennin. Sorry I'm late. Caught in traffic jam in Satan City, you see. I'll get the food ready in no time."
They went in. ChiChi noticed that the table hadn't been set yet. "Where are the kids? They're supposed to set up the table. And where's Videl? This party was her idea. It's not like her to do things halfway."
Presently, Gohan and Videl emerged from their room. They were both wearing only their robes. Gohan saw his mother and their three guests and smiled sheepishly. "Oh, sorry guys. Videl said she had a surprise party prepared for me." He turned to Bulma. "But we really do have a reason to celebrate. I did it! I proved the possibility of time travel. I even got the experimental time travel device completed."
Bulma beamed. "Congratulations, Gohan. I knew you could do it."
Videl put her arms around Gohan and kissed him. "Yes. All that hard work paid off in the end. My Gohan is a genius." She looked around, noticing the kids weren't there. "By the way, where are the kids? They were supposed to set up the table and put up some decorations."
Bulma shrugged. "Haven't seen them. Maybe they went out to get something. Can I see the device, Gohan?"
"Sure. I have it in my lab. As I said, it's only expermental. It works by opening a portal to another timeline. I haven't tried it out yet, since from my calculations it can take subjects to the past or future but I don't know if it can bring them back. As soon as I determine that it can safely bring test subjects back here safely, I'll start testing it. In the meantime I want to set the time to which it will be sent. I haven't done that yet. Only after I do these two things will I really test it with live subjects. Wouldn't be able to bring them back otherwise--"
He stopped when he saw the door to his lab open. Everything was as he left them, except for the experimantal time travel device. The activator button was depressed, meaning it had been used...
And suddenly Gohan knew where #17 and the kids were. Or rather he knew why they weren't here, but not where or when in time they were...
He paled. "Kami sama... What have I done?" Gohan whispered to himself.