"Siam in Love" tells the story of the emotional experiences between two boys who grew up together. Mew and Tong were neighbors and good friends from the same school since they were young. Once, when Tong's family went on a trip to Chiang Mai, his sister Tang got separated with her friends in the forest and disappeared without a trace. Tong's parents felt deeply guilty, and his father indulged in alcohol and neglected his responsibilities. Tong's mother worked hard alone to support the entire family. In order to forget the unpleasant past, Tong moved elsewhere, and Mew lost contact with Tong.
Years later, Mew formed a band called August at the school he attended, and their self-recorded album sold out in the Siam area. Mew's neighbor Ying has always had a secret crush on Mew, and she didn't care even if everyone on the street rumored that Mew was gay.
Meanwhile, Tong, living in another part of Bangkok, had a beautiful girlfriend named Donut. By chance, Tong heard August's songs and liked them very much. However, the album was already sold out. Just then, he happened to see Mew leaving in front of a CD store, and the two reunited.
Mew was creating new songs for a Christmas performance, and his agent hoped he could write a warm love song. But Mew said he had never been in love and didn't know how to write a love song. The assistant of Mew's agent, June, looked very similar to Tong's sister Tang. Tong and his mother brought June home, pretending she was Tang to comfort their father who was addicted to alcohol.
After Mew and Tong reunited, the long-buried emotions in their hearts gradually became clear. Mew wrote a song called "Walking Together" — a love song specifically written for Tong. At the party held at Tong's house to welcome Tang (June) back, Mew performed this song for the first time, deeply touching Tong. Tong finally began to confirm his own feelings, and after the party ended, he kissed Mew, but it was seen by his mom who came to clean up the dishes.
Tong's mom is a Christian who has worked hard alone for many years, not only taking care of Tong who was still studying, but also looking after her husband who had become an alcoholic. She also blamed herself for the disappearance of her daughter. When she discovered the unusual relationship between Tong and Mew, she began to fear that Tong would go down the wrong path and be afraid that her only son would leave her. At this time, her alcoholic husband suffered liver failure due to years of heavy drinking, and under multiple pressures, she finally broke down. She went to Mew and asked him to end his relationship with Tong. At this moment, Ying was listening outside the door...
At the same time, Mew accidentally found many photos of himself hidden in Ying's room and realized that Ying had secretly liked him for a long time. Ying faced Mew very calmly and instead consoled the heartbroken Mew, saying that where there is love, there is hope!
Christmas was approaching, but Mew couldn't sing love songs anymore. Tong was caught between his mother and family and Mew, and ultimately made a choice...