The following version of the book was used to create this study guide: Manansala, Mia P. Arsenic and Adobo. Penguin Random House, New York, NY. 2021. Kindle AZW file.

Lila Macapagal has returned to her hometown of Shady Palms, Indiana. Two factors have played into that decision – her boyfriend's cheating nature and her aunt's failing restaurant, Tita Rosie's Kitchen. Now, Lila is back amidst the smothering support of her aunt, grandmother, three godmothers, and a large extended family. Lila's high school sweetheart Derek Winter is working as a food critic and blogger. Lila discovers that Derek has been ruthless with all the local restaurants, especially her aunt's business. One day, Derek arrives with Mr. Long, who is his stepfather and Tita Rosie's landlord. Lila hates his presence but “sweetly” reminds him that he cannot bring outside food into the restaurant. Derek hurriedly drinks the last of his coffee from Java Jo's the upscale coffee shop next door. A short time later, Derek and Mr. Long have finished their meal. When Lila becomes annoyed that Derek seemed to like the food but still cannot pay a compliment, Derek reacts by falling face-first into his plate. He is alive when EMTs take him away, but Mr. Long calls later with news that Derek has died.

The police investigation begins. Within a short time, the lab confirms that Derek had arsenic poison in his system. Police find arsenic on the dishes Derek was using, but not on any of the other dirty dishes. Soon after, police recover a bag of illegal prescription drugs and cash from Lila's locker and arsenic from a bag of rice in the kitchen. As Detective Park seems unable to focus on anyone other than Lila, she sets out to investigate for herself with the help of her best friend Adeena and advice from family. She learns that there were several people in town who hated Derek because of his articles. She also learns that Derek had been dealing drugs and recently began working as an informant to help police bring down the more important people in the drug world.

Lila also discovers a pattern. Each time Derek wrote a scathing article about a restaurant, the local health inspector showed up, wrote violations, and offered to let them off easier if they hired his contractor. Lila begins to reconnect with some friends from her past while becoming reacquainted with her childhood crush and meeting a new romantic interest who happens to be a dentist.

Lila worries that Derek's mother will believe Lila killed Derek. So, she pays Mrs. Long a visit to clear the air. While there, she and Mrs. Long enter the kitchen and find Mr. Long's body on the floor with a knife protruding from his chest. Lila is horrified when Rosie is questioned about this murder, and she is amazed when Mrs. Long and her sister-in-law Cate want to spend the night with Lila's family as police investigate Mr. Long's death.

Adeena seems to have new information, but a phone call from her is cut short. Lila rushes to the coffee shop where Adeena works to discover that the owner – a man named Kevin who is fairly new in town – killed Derek by putting a fatal dose of liquid nicotine in his coffee. Kevin targeted Derek because Derek had provided drugs to Kevin's sister. An overdose on the drugs proved fatal.

Lila then learns that Mr. Long thought Derek was betraying him over the health-inspector scheme and had been methodically poisoning Derek with arsenic. Mrs. Long figured out that Mr. Long poisoned Derek, and she killed him. Mrs. Long assures Lila she never meant for Rosie to become caught up in the situation and would not have allowed Rosie to actually face charges. Mrs. Long leaves the deed to the restaurant and the coffee shop with Lila and leaves town, urging Lila to live a good life.