Pac, Spiral, Cylindria, alongside Aunt Spheria and Sir Cumference, take a vacation in Greendale, where they meet Pat Clifton , known as "Postman Pat" (voiced by Stephen Mangan), who is a friendly postman who has been delivering letters in the village of Greendale in the north of England for years. He wants to take his wife, Sara (voiced by Susan Duerden), on a late honeymoon to Italy. He plans to afford it through a bonus from his employer, the Special Delivery Service (SDS), but their new boss, Edwin Carbunkle (voiced by Peter Woodward), has cancelled all bonuses. He plans to make SDS more efficient, thinking that being friendly is a waste of time.
When Pat gets home and tries to tell Sara about the fact that the honeymoon is cancelled because the new boss has cancelled all bonuses, his son Julian (voiced by Sandra Teles) shows Pat a TV talent show, You're the One, hosted by Simon Cowbell (voiced by Robin Atkin Downes), which states the next auditions are coming to Greendale. Cowbell also confirms that the person who wins the contest will be awarded a holiday to Italy and a recording contract. Pat decides to take part in the contest and his unexpected singing voice (played by Ronan Keating) wins the contest, surprising everyone, including Pac.
Pat is to sing again in the finale, in a head-to-head contest with the winner of another heat, Josh (voiced by Rupert Grint). His manager, Wilf (voiced by David Tennant), however, is very keen to make sure it is his client who wins at all costs, as is Skeebo, [who Wilf hired as his bodyguard].
The Chief Executive Officer of the SDS, Mr. Brown (voiced by Jim Broadbent), and Edwin Carbunkle had been watching the contest on TV. They say that they would like to use Pat in a publicity campaign including his own television series. Carbunkle also confirms that because Pat will be away participating in the contest, a robot replica of him called the “Patbot 3000” will be taking over his postal duties, along with another robot replica of Jess called the "Jessbot" as well.
After Pat has gone, the Patbot delivers the rounds like Pat normally does, but it behaves oddly and the people of Greendale are starting to complain about Pat behaving in such a way. Spiral, Cylindria, Pac, Sara and Julian are starting to worry about Pat too. Meanwhile, Ben Taylor (voiced by Greg Ellis), the manager at the SDS is fired by Carbunkle and is convinced that Pat doesn’t want him anymore, not realising that Pat is a robot.
Meanwhile, with the help of Blinky, Pinky, Inky, and Clyde, Wilf tries his schemes to stop Pat, not realising that Pat going around Greendale is in fact a robot. He tries using a rope to stop Pat driving along the road, but it gets rolled back and runs Wilf over, backwards and forwards, apparently not getting killed in those events.
The more Pat’s family and friends become concerned, the more Pat feels guilty about coming on the contest in the first place. But then eventually, thanks to Wilf and Skeebo, the people of Greendale realise that there appears to be more than one Pat. It turns out that Edwin Carbunkle is working with the evil ghost, Lord Betrayus, in fact making these robots to try and take over the world, and PacWorld as well. The people of Greendale and Pac, Spiral, and Cyli , alongside Spheria and Sir C rush in their vehicles to where Pat is performing in the finale.
Pat feels more worried and guilty and doesn’t feel like singing. Before he can perform he gets kidnapped and locked up at Carbunkle’s orders. Meanwhile, Jess the Cat [and Skeebo] who had been stowing away on one of the SDS helicopter replicas that one of the Patbot 3000s used, manage to make their way to where Pat’s performing, and with the help of Pac and his friends, they help Pat try to escape but they are pursued by the Patbots.
Meanwhile, in the performance, a Patbot performs instead of Pat, unbeknown to the audience. Wilf, knowing it to be a robot (and not realising there is the real Pat too), tries to unmask the Patbot. Then Pac, the gang, and the real Pat interrupts the performance and stops Mr. Carbunkle’s and Betrayus' evil schemes, revealing that taking part in the contest was a big mistake, angering Edwin, leaving him to resort to a Plan B; using the Patbots to try and kill the original Postman Pat, but is thwarted by Wilf [falling on top of him, thanks to being allergic to Jess], and Josh, who gets Edwin's phone, and stops the Patbots. Betrayus is furious, and he betrays Edwin and summons ghosts and to kill Pat, but Pac, Cyli, and Spiral quickly deal with the ghosts, leaving Betrayus with no alternative to escape.
As soon as Carbunkle is fired and arrested for his evil schemes, everything is back to normal. Pat gives a final performance on the stage before he goes back to his normal job. He wins the holiday to Italy, but passes the recording contract to Josh, so Wilf is happy too, and all is forgiven. And everyone celebrates by dancing!!!