This book is part of the "Dawn of the Clans" arc. This story mainly shows us how the tradition of meeting at four trees are formed and the basic of the warrior codes are made. 
At the start of the book, Gray Wing and his brother Clear Sky was divided into two different groups, and there is a great tension between the groups. One of the young cats was able to foresee the future and can tell there are troubles ahead for both the groups.
Thunder, Clear Sky's son, was angry at Clear Sky for how cruel he is and how he kept his boarder so strictly at many cats' lives, and decided to leave Clear Sky's group. Thunder went and found Gray Wing, being the second time seeking refuge at Gray Wing's camp.
At the meantime, Clear Sky was strictly monitoring cats in his group and seeing if they are loyal, claiming that when hard time strikes, only loyal cats can live through the cold season successfully, and they have to all unite. At this time, a cat named Fallen Leaves, who had came from the mountain with Clear Sky and had a strong connection with Clear Sky before, was threatened and punished for even the slightest question she raised against Clear Sky, showing how Clear Sky do not care about their past connections and only care about their loyalty now, further proofing Thunder's previous decision to leave is reasonable.
Gray Wing watched all these happen from afar, and reacted to the hostile change in Clear Sky's group. He saw that cats were planning to learn battle moves, so Gray Wing decided to start training their own cats to fight like warriors. At this point in the story, the warrior code is still not established, and even kits are participating in fighting training, and this was seen by Clear Sky. Clear Sky did not realize that Gray Wing only trained his cats in response to his own cats doing battle training, so decided to go even harder on his own cats in training, further raising the tension between the groups.
Later in the story, Wind Runner and Gorse Fur, who had both lived with Gray Wing's group for a long time and gave advices to the group often, was officially welcomed as part of the group. After they are officially welcomed, the cats start bringing up the idea of talking to Clear Sky. Some cats are hesitant if Clear Sky will listen - especially Jagged Peak. But Gray Wing is certain that Clear Sky will listen to him - because after all, they are brothers. And everything that they'd been through must bind them together, and not push them apart.
Back at Clear Sky's camp in the forest, Thorn and Dew find Clear Sky. They convince Clear Sky that they're capable of joining - and tell them that they know what goes on in Gray Wing's camp. They tell him about Wind Runner and Gorse Fur's warrior ceremony, although not knowing it was the ceremony and not stating it. Clear Sky starts putting pieces together - and gets the assumption that Thunder was spying on Clear Sky and his cats to prepare Gray Wing for a battle. Clear Sky is infuriated, and knows that there'll be war between him and his brother. Clear Sky looks to the moor, and asks himself if Gray Wing wants battle. And he'll give him better than that. He'll give him war.
Soon, Gray Wing decided to send Jackdaw's Cry and Jagged Peak to invite Clear Sky to a talk at Four Tree a few days later. Clear Sky agreed, but holding Jackdaw's Cry as hostage, starving him, and propose to bring many of his trained cats to the meeting. This foreshadowed how the meeting didn't go well and eventually a fight broke out.
Basically, mass amounts of cats fighting and dying happened during the meeting, and both Clear Sky and Gray Wing are sorry for starting this battle, eventually declaring peace with each other.
After the battle, many dead cats appeared as spirits and gave guidance to the cats, this marked the start of the group meeting every full moon at the Four Trees, which eventually turned into clan gathering in the future.