Join In! Rivals

'''Join In! Rivals' (参戦！らいばるたち, Sansen! Rai barutachi) is the tenth episode of the first season of Isekai Quartet'' and tenth episode of the full series overall. It aired on June 11, 2019.

Summary
It's time for Field Day, and Roswaal hints that whoever wins will get to go home. But then they meet their opponents…

Plot
Roswaal announces to Class 2 that Field Day is coming, eliciting confusion to Subaru as the class already had a field trip. Roswaal clarifies that it will be a competition that they will have to win. To add enticement he informs that if they win the competition the class may be able to return home. This remark catches the attention of the entire room. But Subaru interjects, asking who will they be competing against, as Class 2 is the only class group in the entire room. Roswaal reveals that Class 2 will be competing against Class 1, in the room next door. Subaru running oversees through the window, that the next class to be filled with other characters.

Subaru is especially shocked at seeing Reinhard in the class. The teacher, Vanir, politely asks Subaru to refrain from disturbing his class. Roswaal, Tanya and Ainz come over to see before the former reminds them to return to class. Ainz and Tanya know that there wasn't a class there before. Roswaal attributes it to how the world works.

After class Tanya, Ainz and Subaru discuss the people in Class 1. As Ainz sees that the Pleiades are in Class 1, and Subaru's comrade Reinhard is there as well, Tanya deduces that other people from their worlds are trapped in the current world. Since Roswaal stated they can go home if they beat Class 1, Ainz suggests they could comply. Tanya asks about the strength of Class 1. Ainz is confident that the combat maids will be no issue. They ask Subaru to tell them more about Reinhard, only for him to state that the young man possesses Divine Protection that is insanely powerful. They hear a commotion outside and see Aqua recognizing one of the students from Class 1, Chris. Kazuma and Darkness also greet Chris. Aqua demands why Chris isn't doing anything to stop this only for Chris to state she is not in control of the situation. Aqua finds it amusing that Chris has no idea what is going on, only for Darkness to be annoyed for Aqua insulting her goddess.

In the end Aqua earns a blow on the head, causing her to break out in tears. Chris tells Kazuma and Darkness that they will be rivals now and to be careful as they cannot be revived in this world. Subaru hearing that wonders about his [Return by Death] ability that allowed him to come back to a saved point in his life after he is killed. Subaru is hesitant to tell his new friends his ability but decides its worth a shot. To test it out he covers Tanya's ear and tries to explain his ability only for him to stop when he feels death looming for his penalty. Realizing it is still in effect he stops, only to be run down by Hamsuke and a Death Knight due to his ability to attract magic beasts.

Class 2 gathers that the student body delivers the plan to win and return to their worlds. The only one to object is Kazuma as he finds the current world comfortable. Kazuma's friends berate him for his selfishness and reminding him that he still needs to defeat the Demon Lord in his world. Kazuma rebukes that there is no way he could accomplish that and prefers to be lazy. The other classmates find that disagreeable that they declare they have no intention of losing. However, Kazuma still refuses to help, prompting Tanya to offer him an 'initiative' to motivate him, which mostly involves forcing Kazuma to run for his life as the solider and Puck bombard him from the air with magic.

Trivia

 * Tanya has Kazuma run while carrying a shovel just as she does to her unit when training them.
 * The Witch of Envy Satella from Re:Zero did not appear officially in this episode. Due to an unexplained circumstance, she was present in the new world and squeezed Subaru's heart to stop him from telling anybody (especially the characters from the other series) his "Return by Death" ability.