Time traveler Blendin is back, and he wants revenge against Dipper and Mabel!
For the twins, we call that “Thursday”.
Yes, in the future, Blendin was punished by the giant Time Baby (just roll with it) but he escaped from jail and demanded some sort of future combat against the twins. That works to maybe get him out of jail.
The kids are actually distracted as it is by the fact that it is Soos’ birthday, something they quickly learn he doesn’t like. Birthdays upset him. Why? No one knows. A surprise party brings tears. Maybe if the kids take him to play some laser tag. He apparently likes that, but Dipper and Mabel get zapped to the future first. The rules? They enter an arena with Blendin and the winner gets to both decide the fate of the loser and gets an all-powerful “time wish”.
That, uh, sounds scary since Blendin wants to erase the kids from existence should he win.
A little quick thinking gets Dipper and Mabel zapped back to their own time…mostly. They arrived ten years too early, leading to some fun sites of Gravity Falls in the 90s. A five year old Wendy has a little crush on Dipper. Robbie is a jerk in any age. And Soos is having a birthday party. But he’s upset. Why? His father didn’t show. Again.
That’s why he hates birthdays.
Seeing that, the twins opt to go back to Blendin’s time, figuring if they win then Soos can see his father. How can it go wrong? There are two of them and one of him.
Oh yeah, he’s been practicing in jail.
But the twins win when it comes down to the last contest. What is it? Laser tag. First person to grab an orb wins, so when Blendin starts gloating, Dipper zaps him a couple times while Mabel grabs the orb. The twins win. And though Mabel initially calls for death, the twins ask Blendin get reinstated to his old job since it was their fault he lost it in the first place. Then they give Soos the wish, but he doesn’t wish to see his dad. Family cares for you, and Dipper and Mabel battled through time and space to get him a time wish, so he wishes for them to get cleaned up (Blending doesn’t think that’s worthy of a time wish) and then a slice of infinite pizza that will never run out (Blendin approves).
So, now Soos likes birthdays.
Also, in the past, we learn he got his job on his 12th birthday when he found a screwdriver Dipper had dropped and Stan hired him on the spot as the new handyman.
So, I guess that mystery is solved.
If it was a mystery.