The Siege of Serus is the 32nd episode of The New Adventures of He-Man.
Hydron and Sagitar in the Starship Eternia warn the other ships about an incoming asteroid shower. Due to Primus’ shield however, these asteroids simply burn away. In their lab on Primus, the Scientists wish that one of the asteroids would get through, as they could study it. It appears as though they get their wish, and one of the asteroids is able to pass through the hole in the shield. Hydron and Sagitar go after the Asteroid, and open fire on it. To their shock, their lasers have no effect, and the asteroid is now headed straight towards Levitan.
At Master Sebrian's home, Adam transforms into He-Man and races off to intercept the asteroid. He-man tries to use his sword on the asteroid, but it too has no effect, The Sorceress contacts him and advises he use the asteroids own power against it. He-Man then creates a beam of energy which directs the asteroid into the sea. Meanwhile, the Scientists watch this, and all decide they want to investigate the Asteroid for themselves. Alcon believes he can sneak away by himself, only to find his fellow scientists have had exactly the same idea, as they all meet in the lift.
Sebrian then contacts them, and tells them the asteroid is causing trouble by the Underwater city of Serus. He-Man, Hydron and the Scientists then set off for Serus. Hydron explains to He-Man that Serus is his home city, and that he was the only member of his family who left to join the service, he is also worried about his sister Aquata. Aquata contacts them, to Hydrons relief, and tells them the asteroid is causing flooding in the underwater city, which is getting worse.
He-Man and the others arrive in the Astrosub just in time to save Aquata, and seal the cracks in Serus’ shield with Gepple's Goo.
Aquata thanks them for their help. Hydron says all he did was pilot the ship, to which Aquata says he always liked to give other the credit. He-Man tells her Hydron is known as a brave warrior throughout the galaxy, and Aquata says everyone in Serus is proud of him.
He-Man uses his sword to seal the cracks in the shield, with the help of Gepple's Goo. The Scientists find a sample of the asteroid rock, and discover that not only is it stronger than anything else they have ever encountered, it is alive. As they speak, a mysterious voice is heard, and an odd creature has appeared on the view screen. The creature introduces itself as Keto, a living asteroid who crash landed on Primus. Keto tells them that to continue with his travels, he and his people will drain all of Primus’ energy, destroying them.
They track Keto to an old underwater science station. He-Man and Hydron arrive at the station, and He-Man tells Hydron to stay behind, if he does not return, he tells Hydron to blast the station over the edge of the abyss. He-Man goes inside the station, wearing a helmet.
He-Man explores the station while Hydron watches his scanner with He-Man as a red blip. The base begins to crack apart, and water comes gushing through, He-Man runs as fast as he can to avoid being drowned, finally redirecting the water by tearing open a door. He then tells Keto ‘Is this how you do battle? You fight like a coward!’, just then, the base begins to crack apart again and Keto rises up from the floor, prompting He-Man to backtrack and say ‘Perhaps coward was bit too strong a word.’
He-Man tries to use his sword against Keto, but again it has no effect. Realizing Keto wants this energy, He-Man offers it to Keto, if he leaves Primus in peace. Keto then reveals that his true mission was to seek out planets that may be threat to his people, and He-Man's self-sacrifice a moment ago has shown him Primus is no threat, and will survive. He-Man tries to convince Keto that learning to tolerate others and use compassion is a better way, to which Keto responds compassion is a quality his people do not possess, and perhaps that is what makes the people of Primus more powerful than them. Keto leaves Primus, drifting back into space, speculating that perhaps, they can learn from He-Man, and Primus.
- This is the first episode of The New Adventures of He-Man not to feature Skeletor.