The Angels' Second Advent is the first episode of the second season of Mobile Suit Gundam 00
A space type Tieren and Realdo engages a mobile suit equipped with orange particle GN Drive. Two others detonate a bomb in an asteroid base of the anti-government resistance base Katharon. The suits then speed off to Earth. The report of this defeat is sent to Klaus Grado and Shirin Bakhtiar, who consider speeding up their operation at Proud.
Meanwhile on colony Proud, Saji Crossroad is starting on his job as a space engineer. Suddenly, one of his co-workers Eddie Miyasaka is arrested by the Federation secret police, the A-Laws for suspicion of being a Katharon member. Saji is also arrested for no apparent reason. They are put to work under high gravity, with their wardens wearing protective suits. Eddie asks if Saji is part of Katharon; he firmly states no. Eddie says it's his lucky day, as something is about to happen soon.
Meanwhile, what appears to be a new version of the CBS-70 Ptolemaios, Mileina Vashti informs Tieria Erde that an A-Laws detachment is heading for Proud. He goes out in GN-008 Seravee Gundam, knowing that a certain someone will be there.
At Proud, someone in a black space suit sneaks inside the colony. He disables the suit of a guard, demanding to know what's going on; all the what guard can say is that an evacuation order has been given; the black suited person then leaves. Outside, Katharon forces consisting of a number of Realdos and a Virginia class cruiser, attack the colony, taking out its defenses. They then hack into the gravity ring, stopping its movement and taking off the heavy gravity. Katharon forces then disembark to rescue the prisoners. Unfortunately, an A-Laws cruiser, with Louise Halevy among them, deploys two GN-X III and an Ahead to destroy the vessel and any mobile suits. They then deploy the automatons, four-legged robots that massacre the escapees. Saji is saved by Setsuna, who uses explosive gel against automatons chasing them. Once they get to a hanger, Setsuna reveals he's a Gundam Meister and takes off in the GN-001RE Gundam Exia Repair. He proceeds to destroy all the automatons in the colony. Unfortunately, the battle-damaged Gundam is no match for the new A-Laws suits, which the Ahead qtakes out his sword and a GN-X III goes for the kill. Setsuna gets a lucky save by Tieria, who uses his new Servavee. A GN-X III tries attacking from behind only for a GN Field to activate and a chest sized Gundam face boost its strength as Seravee slices the ambusher. The new CBS-74 Ptolemaios 2 arrives to pick them up. In the Proud's hanger, Saji confronts Setunsa and picks up a gun against him, believing that Celestial Being was responsible for his sister's death and Louise's injury. He demands that Setsuna give everything back to him.
On Earth, reports of Celestial Being's reappearance is all over the world; at a Katharon base, Shirin is concerned about receiving a military intervention, but Klaus is optimistic that they and Celestial Being share the same goals; Ribbons Almark meets with Wang Liu Mei.
Aboard the Ptolemy 2, Saji is imprisoned, as Tieria questions him as to why he didn't kill Setsuna; he responds that he would just end up like them; killers. Tieria berates him for not seeing what the world is truly like. Setsuna observes his new Gundam, 00 Gundam. He later goes down to Earth to recruit Lyle Dylandy as the new Lockon Stratos.