Mrs. Montague and Mr. Capulet (Julie Walters and Richard Wilson) are two elderly people who despise each other. When they leave the garden, objects come alive in both their gardens. The Montague garden is filled with blue garden gnomes, and the Capulet garden houses red garden gnomes. Later, both blue and red gnomes attend a lawnmower race. Representing the blues is Gnomeo (James McAvoy) and representing the reds is Tybalt (Jason Statham). During the race, it looks like Gnomeo is winning; however, Tybalt cheats and wins the race, destroying Gnomeo's lawnmower. Gnomeo and his best friend Benny (Matt Lucas) insult Tybalt for cheating, but Tybalt ignores them.



Later that night, Gnomeo and Benny, wanting revenge, infiltrate the red garden in disguise, intending to spray the reds' lawnmower with blue spray paint. With Tybalt sleeping and Tybalt's best friend, Fawn (Ozzy Osbourne), playing cards with small red gnomes, it looks like there are no flaws in their plan, until Benny foolishly sprays Tybalt's well and accidentally triggers a security light in the process, alerting the red gnomes to attack. Fortunately, both escape; however, Gnomeo ends up in a different garden. He then bumps into the daughter of Lord Redbrick (Michael Caine), who is called Juliet (Emily Blunt). Juliet is looking for the last remaining orchid in the greenhouse, planning to use it to beautify the red garden. Gnomeo is intrigued by her, and reaches for the orchid as she is about to complete the garden, and they fall in love in the process. During this time, they discover they are from opposing clans. When they both go back to their gardens, Juliet tells her friend Nanette (Ashley Jensen) about her newfound love, much to her surprise. Nanette states that the relationship is romantically tragic.



Gnomeo and Juliet then have secret meetings in a secret garden, where they meet a pink plastic flamingo named Featherstone (Jim Cummings). He supports and encourages their love, and the two begin to meet regularly. Though when the two of them are getting ready for a date, Juliet's father introduces her to Paris (Stephen Merchant), a red gnome that Nanette has fallen for, though Juliet manages to get away.



Later, when the two return back to their gardens, Gnomeo finds his mother (Maggie Smith), who is distraught after the reds infiltrated the garden and destroyed the plant Gnomeo's deceased father planted. The blues want Gnomeo to take revenge on the reds, and he realizes that he cannot refuse unless he tells his secret. He tunnels underneath to reach the red garden, but just as he is about to spray the prized flowers of the reds, Juliet sees him. He backs out suddenly, telling Benny that the nozzle on the spraying bottle was jammed.



When he and Juliet meet up again, they briefly argue until Featherstone stops them, telling them that other peoples' hate destroyed his love. He and his girlfriend were separated when the two people living in the house, where their garden was, got divorced. After he has explained this, Gnomeo and Juliet apologize, but when they are about to kiss, Benny sees them, distracts them, then runs into the alleyway, where Tybalt is waiting with his lawnmower. Tybalt drives at Benny and knocks his hat clean off. Tybalt fights Gnomeo on his lawnmower, but he is destroyed when crashing into a wall. The reds attempt to attack Gnomeo, but Juliet, to the surprise of her father and clan, defends Gnomeo, saying that she loves him. A woman suddenly jogs along, so all gnomes become still and inconspicuous. Gnomeo ends up on a road, and everyone believes he was run over by a truck. Juliet's father then glues her to her fountain because he does not want to lose her like her mother. Gnomeo's pet, a mushroom named Shroom, is left alone and goes on the road, where he realizes that what appeared to be Gnomeo is actually a broken blue teapot, and that Gnomeo is still alive. Gnomeo eventually ends up in a park, and climbs onto a statue of William Shakespeare (Patrick Stewart) and tells him his story. Shakespeare then tells Gnomeo that his story is very similar to Romeo and Juliet and that it is likely Gnomeo's will have a sad ending as well. Shroom and Featherstone come to find him.



Benny, meanwhile, buys a large lawnmower online, called the Terrafirminator, to get revenge on the Red Gnomes, despite Shroom trying to convince him that Gnomeo is still alive. The Terrafirminator goes out of control and destroys most of the two gardens. Gnomeo makes it back to Juliet to try to un-glue her, but he is unable to. She tells him to go, but he refuses, and the two share a kiss just as the lawnmower crashes into the fountain, self-destructing in the process. Everyone believes that both Gnomeo and Juliet are dead. Lord Redbrick and Lady Blueberry, both realizing that their feud was responsible for this, decide to call a truce. Suddenly, both Gnomeo and Juliet climb out of the rubble and are both fine. The movie ends happily with the two of them getting married on a purple lawnmower, which symbolizes the truce.