Nobby, a sweet but dim-witted football fan living in the impoverished town of Grimsby, has spent decades searching for his long-lost brother Sebastian. Unknown to Nobby, Sebastian is actually a top MI6 agent involved in a mission to thwart a global terrorist plot. When the brothers eventually reunite, their polar opposite lifestyles and personalities lead to comical misunderstandings. Despite Nobby's well-meaning but often disastrous attempts to help, Sebastian finds himself increasingly entangled in chaotic situations, complicating his mission further.

Their reunion forces Nobby and Sebastian to go on the run together across multiple countries, dodging both terrorists and law enforcement. Along their journey, Nobby's unrefined skills unexpectedly become useful in their survival, giving Sebastian a new appreciation for his estranged brother. As they work to uncover the mastermind behind the plot, the brothers must navigate numerous perilous and absurd situations. Along the way, they confront personal demons and rekindle their familial bond, culminating in a high-stakes confrontation that could determine the fate of millions.