Set against the backdrop of Malaysia’s favourite sport, Badminton, Summer Brothers chronicles the lives of long lost brothers, Wei Zhong and Ah Di, played by William and Coby, who were separated after the divorce of their parents 20 years ago. Ah Di who was diagnosed with mental disability when he was young, lives a poor but happy and peaceful life with his mother, Shu Xian (Remon). When Ah Di’s mother finds out that she is about to die from her terminal cancer, she decides to unite the brothers. However, she lost Ah Di during their journey to Kuala Lumpur in search of his older brother. Ah Di is stranded but made new friends with Xiao Mei (Lim Jing Miao), a kind and naïve girl. With the uncanny rapport between them, the two become instant bosom buddies.
Xiao Mei’s parents died in a car accident and both her and her younger brother, Yu Cheng (Adrian) are forced to board at their relative’s home. The siblings suffer through their dependence on others, but for the sake of his older sister, Yu Cheng endures the hardships thrown at him. Yu Cheng is a top student who has just graduated from school, but in order to make ends meet, he is forced to quit his studies to work. His only wish is none other than gaining independence to support his sister. Ah Di is envious of Xiao Mei and Yu Cheng’s relationship, which makes him reminisce the childhood spent with his brother.
Meanwhile, Ah Di’s mother manages to locate Wei Zhong and informs him about Ah Di’s disappearance. Wei Zhong is not only unmoved by the news, but has no intention to reunite with his brother. Many years ago, Wei Zhong encountered an accident that caused an irreversible injury, forcing him to give up badminton forever, while allowing his main rival and best friend, Qi Jun (Johnson) to not only step up but also end up marrying his college sweetheart, Chen Rou (Stella).