Summary (courtesy of Herms):
Zamasu has brought more tea for Gowasu, only to discover Beerus and co. there with him. He politely greets them all in turn (even recognizing U7’s Kaioshin on sight), but wonders to himself what they are all doing here. Do they suspect anything…? He asks them if they ever found out anything more about the mysterious Time Ring-wearing person they were investigating, but Beerus says they’ve come back for another reason: Goku wants a rematch with Zamasu, and won’t shut up about it! Goku starts to deny this, but Beerus cuts him off with a quick blow to the head.
Zamasu is now even more suspicious, but merely says he was likewise unsatisfied with their first match and would gladly have another. Shall they start now? Beerus says he’d hate to intrude on their tea time, and anyway, it’s just about time for his afternoon nap. They’d better go, and save the fight for another time! Before they leave, Whis presents Gowasu with some daifuku (rice cakes stuffed with bean jam) as a gift.
Rather than go all the way back to Earth though, Beerus and co. stop out in space, using Whis’ staff to watch Gowasu and Zamasu’s interactions. Beerus could tell from their brief meeting that Zamasu is filled with the urge to kill Gowasu. Goku (and Gowasu himself) aren’t able to sense it, but being a God of Destruction, Beerus is particularly sensitive to that kind of thing, as is Whis. However, they’ve got to wait for concrete evidence; they can’t destroy U10’s apprentice Kaioshin simply for having the urge to kill someone. Goku’s worried though that once they get proof it’ll already be too late.
In Trunks’ alternate future, Black and Zamasu are hanging out together. Black boasts that next time he’ll finish Goku off for sure, but Zamasu says doing so would leave Black without any more “prey” to grow stronger. Anyway, they’ve proven Goku is no longer a match for them. Zamasu is pleased with his immortality, and says perhaps Black should likewise use the Super Dragon Balls to become immortal. However, Black is only interested in growing stronger: they need his “infinitely” rising power to fulfill their plans. Zamasu finds it ironic that the “0 Mortals Plan” relies on a mortal’s power.
Back in the present, Bulma works on the time machine while Future Trunks mopes about on his own, sad about having run away again, leaving Mai behind. Pilaf and co. watch him from afar, noting that he’s been like this ever since returning from the future. Pilaf thinks now that he’s sad and vulnerable it’s the perfect chance for Kid Mai to make her move, but she hates the idea, saying that Pilaf doesn’t understand Trunks’ feelings. Kid Trunks approaches his future self, and chews him out for getting so upset over one lost fight. He won’t be able to protect Mai like this! He continues taunting Future Trunks and goading him to fight. Going Super Saiyan, he knocks Future Trunks down, yelling at him to get up again. “You call yourself a Saiyan?!”
In U10, Gowasu finds that daifuku goes along great with tea, and urges Zamasu to try some, but he politely declines. Zamasu says that he’s thought more about what Gowasu said about the balance between good and evil, and agrees with him now: sometimes evil can result in a greater good. He circles around behind Gowasu, and Goku and co. (still watching on Whis’ staff) can see that he’s forming his hand into an energy blade, preparing to strike! Goku urges Beerus to act quickly, but it’s too late: Zamasu cuts Gowasu down.
However, Whis immediately reverses time back to before the attack. Now they have all the proof they need to intervene. This time when Zamasu goes in for the kill, he finds his hand covered in a cat-themed mitten (courtesy of Whis), which nullifies his attack. Beerus and co. appear and explain to Gowasu how Zamasu was trying to kill him. U7 Kaioshin explains in detail: Zamasu wished to kill Gowasu in order to steal the Time Rings, use the Super Dragon Balls to become immortal, create an imitation of Goku, and destroy all mortals.
Gowasu is shocked to hear that Zamasu hasn’t actually turned over a new leaf, but Zamasu says he has no need to: he’s simply trying to enforce divine justice, after all. Still, he’s surprised they could have worked out his entire plan simply from the fact that he wanted to kill Gowasu. Goku says they saw Zamasu’s plan in action in the future: tons of destroyed cities and dead people. Zamasu is happy to hear that his wish becomes a reality. That means he can’t die here then! But Beerus tells him to not get carried away…and promptly annihilates him in a single blow.
Back on Earth, Kid Trunks and Future Trunks conclude their battle. Future Trunks understands how his kid self feels; he’ll win now for sure! Suddenly, Goku and co. crash back to Earth, and Beerus chews Whis out for another rough landing. This is why Goku wanted Kaioshin to take them back! They fill Future Trunks in on events, and Beerus assures him that with Zamasu dead, Black will be gone too, and his future will have returned to normal. Trunks finds this questionable though: after all, his alternate future didn’t change when they defeated the androids over in this timeline.
Beerus tells him that’s just mortal logic: when a god destroys another god, it will definitely interfere with time-space! Bulma likewise wonders if Beerus is right; will Trunks’ future really change? Goku says if Beerus said so, it must be true. Trunks hopes so, but still has doubts…