In the heart of Silicon Valley, Ben has to stop Victorian villain Steam Smythe from securing an epic power source sealed in a Tesla time capsule.
The Tennysons arrive at Clara Vista, and Gwen is excited. Ben is confused on why they care about this, and Gwen tells him that they are digging up a time capsule left behind by Nikola Tesla. Ben tells Gwen that he's all about the future, and that history is boring. Max walks in and Gwen tells Max to tell Ben how important the past is, but Ben transforms into XLR8. He thinks he can go back in time, and stop Nikola Tesla from being born, but Gwen tells him he should stop stepping in poison oak first.
At the town, the announcer talks about the history behind the town. Two guys dig it up and they prepares to open it up. Ben thinks he sounds kind of weird, but Max tells him he was a brilliant scientist. Steam Smythe flies in, and prepares to steal Nikola Tesla's invention, and destroy the valley. The two boys who dug up the capsule, reveals themselves to be Steam Smythe's Mechanoids. Smythe and his Mechanoids fly off. Ben transforms into Heatblast. He shoots fireballs at the robots, destroying the two holding the capsule. The capsule falls into the lake, and Heatblast jumps in to get it. The capsule is pulled out by Smythe and causes Ben to fall towards land, and time out.
Gwen and Max arrives and questions if Ben is hurt, he states only his pride is. He apologizes to Gwen for not being able to save the capsule, but Gwen thanks Ben for trying. Max comes in and tells them it isn't over yet, and they still have to save the valley. Ben comes up with a plan, to board Steam Smythe's blimp with the Rustbucket. Ben transforms into Upgrade and Upgrades the Rustbucket.
On Smythe's blimp his robots bring him the capsule. Steam Smythe tries to be gentle, but then the Rustbucket flies in. The Tennysons board the ship. One of Steam Smythes bots starts talking, confusing him. It reveals itself to be Upgrade upgrading a Mechanoid. Ben takes out most of the bots. The bots enter the RV, and Max takes one out. Max gives Gwen a bat, and they both take out Mechonaids. Upgrade and Steam Smythe fights over the time capsule. Ben eventually times out. Smythe's Mechanoids grab Ben, while Steam Smythe opens the capsule. It's revealed to be a teddy bear, and disappoints Steam Smythe. Smythe plans to take out the town block by block. Ben kicks the teddy bear into the gears of Smythe's blimp. Ben runs to the Rustbucket and tells Gwen and Max to get inside. Max drives them off of the blimp. Gwen asks Ben if he has a plan, but his watch needs more time. The watch times in and Ben transforms into Four Arms. Four Arms jumps down and catches the RV, and prevents it from falling off a cliff.
The tri returns to the damaged blimp. Max questions if Steam Smythe survived, but the Tennysons doubt it. Steam Smythe flies out, telling them that he will return, and will rely solely on his own inventions. Ben believes they made history, to which Gwen recalls that he doesn't like history. Ben admits the past is dumb, and he cares about the future. He also thinks Gwen is baller. Max joins in and states together they can make the future bright.
Ben: History is old, boring. As uncool as Grandpa's shirt. Max: Hey, a psychic predicted this shirt would be very popular. 'Course, he's the one who sold it to me. Gwen: Oh, come on! We can learn alot from history. Grandpa, tell Ben how important the past is. Max: It's very important, and some of us have more past than others.
Gwen: Ben! Are you hurt? Ben: Only my pride. I'm sorry, Gwen. Gwen: You gave it a great shot. Thanks. Max: It's not over. We need to save Silicon Valley. Ben: Hey, saving the world's what I do best, right? Grandpa, I know how much you love the rustbucket. Max: It's my favorite thing in the world, but it is just a thing. Ben: Good, 'cause here's the plan. We're gonna board that blimp with the bucket. Gwen: Uh, were you hit on the head? That would mean the rustbucket would have to fly. Ben: You're right Gwen -- 100% right. Gwen: Ben said I'm 100% right. This can't be good.
Steam Smythe: Now, I must be gentle. We don't want to destroy Tesla's beautiful gift to us. (Rustbucket flies in) Eh? What is this abomination? Max: Go on, insult the Rustbucket. I dare ya.
Max: You think Steam Smythe made it? Ben: Eh... Gen: Well, what's that? Steam Smythe: We will meet again, and next time, I'll rely solely on my own inventions! Mr. Schmoopie indeed! Ben: What a sore loser. Looks like we saved the day and made a little bit of history ourselves. Gwen: I thought you didn't care about the past? Ben: Gwen...the past is stupid. There's too much steam and everybody has big, weird mustaches. But I do care about you. I think you're baller. And I care about the future. Gwen: Baller?! Max: Together, we can make the future bright. (Eagle lands on Max's shoulder)(Eagle shrieks). Gwen: Gee, Ben, I guess I think you're baller, too.
The episode's title is a reference to the idiom "mum's the word".