At the beginning of the series, 15 children (eight males and seven females) are spending some time on the beach. While they're at the beach, they discover a cave that they go into and explore. During their exploration, they find an area with working computers and electronic equipment. Before the kids can leave, the owner of the electronics returns. He claims his name is Kokopelli, and that he's a programmer working on a brand new game. He says that in the game, a large robot has to defend Earth against 15 alien invasions. He manages to convince all but one of the children to test the game and enter into a contract. After they agree, the kids mysteriously awaken on the beach, and night has fallen. The kids start to wonder if they all had a strange dream.
As the kids return to where they're staying, two giant robots suddenly appear. Kokopelli is inside one of the robots and is controlling it. During this first battle, he provides a brief tutorial to the children as to how this robot operates. The children watch as Kokopelli destroys the enemy robot. After he is finished, Kokopelli tells the children they are on their own and sends them back to the beach. As they are being teleported, one of the children sees Kokopelli whispering, "I'm sorry."
Bokurano has a very interesting premise behind it. However, the first episode suffers from the fact that there is a large cast of characters, and it's hard to remember who is who. And since it's hard to get to know the characters very well, it's hard to feel much in the way of empathy when it comes to their situation.
When it comes to the animation, I thought the backgrounds and the characters were very well-drawn. The major weakness in the animation is found in the computer graphics used for the robots. Unfortunately, the computer animation just stands out too much from the 2D animation and draws too much attention to itself.
When I watched Bokurano, I didn't really see anything visually that would be objectionable for younger viewers. However, with the kind of storytelling being used, and with the majority of the characters being middle school age, I would recommend Bokurano to anime viewers who are 13 or 14 years of age and older.