WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 184, "Puppets," now streaming on Crunchyroll.
The Boruto anime is slowly bringing the threat of Kara much closer to Konoha than Naruto and Sasuke would like now that Jigen's airship has crashed outside the Hidden Leaf. To make it worse, the village has been betrayed by Ao, who's been sent by Koji Kashin to retrieve the Vessel the terrorists need for their secret project.
Sadly, with the Leaf shinobi caught off-guard in Episode 184, the traitor's able to brutally murder the heroic ninja Mugino, making it clear that Kara will stop at nothing to achieve their goals.
When Boruto, Sarada, Mitsuki and Katasuke arrive at the crash site, they're attacked by a bunch of robotic puppets. They eke a victory out thanks to Katasuke's scientific tools, but there's no sign of Konohamaru or Mugino. Boruto gets worried as Naruto told him they lost contact with the duo, but thankfully Katasuke has brought Chamaru, a puppy from Ryutan who's really impressive at tracking.
They're able to quickly locate Konohamaru and Mugino, but the latter's injured after battling the puppets. But before they can formulate a proper plan to get the intel from the airship's mainframe back to Naruto, Ao enters the cave with a giant chain gun. The group's surprised to see him, thinking it's a coincidence because they have no idea he's there on behalf of Kara. At this point, he indicates that he's no longer a shinobi and he needs all the info they have regarding the Vessel.
When Ao realizes he's going nowhere with the discussion, he begins firing upon them, only for Boruto's Rasengan to damage his gun. But Ao's as resourceful as ever, using Katasuke's jutsu absorption tool and guns hidden under his clothes to knock the battalion out. Despite his age, he still has that ruthless edge to him, but Mugino catches and grabs him from behind. He reminds Ao of when they met at the Remembrance Day for those who died in the Kaguya War and how the veteran offered him words of inspiration.
Alas, it was all part of Ao's facade. With his robotic arm pressed against Mugino's stomach, he unleashes a fire-blade from his elbow that goes straight through, impaling Mugino and leaving Boruto shattered. Ao makes it clear that he's not a ninja -- he's a tool for Jigen. This forces Mugino, in his final act, to sacrifice himself to allow the others to escape.
Mugino deploys an Earth-style jutsu that brings the cave down on him and Ao, allowing Boruto and co. a chance to escape. They reluctantly scurry off to complete the mission but as Ao gets up from the rubble, he vows to finish them off. It's a very tragic end for Mugino, leaving Boruto thirsty for revenge as Mugino was a very important mentor to him.