Trama

A family of five heads to Granddad's big 75th-birthday party at an uncle's estate in rural Scotland. The parents are separated and hope their three kids won't mention it. The kids love, can talk with, and would do anything for Granddad.

Doug (David Tennant) and Abi (Rosamund Pike) take their kids on a family vacation. Surrounded by relatives, the kids innocently reveal the ins and outs of their family life and many intimate details about their parents. It's soon clear that when it comes to keeping a big secret under wraps from the rest of the family, their children are their biggest liability. Find out how the rest of the family cope and see if the holiday will end happily.—Lionsgate

Newly separated Abi [Rosamund Pike] and Doug [David Tennant] and their kids 10yo Lottie [Emilia Jones], 7-8yo Mickey [Bobby Smalldridge], and 4-5yo Jess [Harriet Turnbull] are driving from London to Doug's father's Gordie's [Billy Connolly] home in the Scottish Highlands for Gordie's 75th birthday party, believed to be his last, as he's dying of cancer. Because of Gordie's condition, Doug and Abi don't want to tell him that they're divorcing, so they swear the troubled kids to secrecy.

Mickey has a fascination with Norse mythology, and it turns out that Gordie recently discovered that he's "84% Viking". Meanwhile, Doug's tight-ass brother, Gavin [Ben Miller] has planned a grand party with 215 guests for his father, complete with catered meal and a live Celtic band.

The day of the party, Gordie, who shuns displays of wealth and class, announce's hes taking Doug and Abi's kids on an all-day outing, promising to have them back by 7 p.m., when the party is to begin. He packs them into his pickup truck and drives them to a friend's petting zoo, after which they drive to his favorite spot on an isolated beach.

While the kids frolic, Gordie dies. When they try to awaken him and realize he's dead, Lottie instructs Mickey and Jess to stay with him, and runs back to the house for help, where every adult she approaches is quarreling. Despairing of getting any of the grownups to stop their immature behavior and pay attention, she grabs her grandfather's prized possessions and returns to the beach, where she and her sibs build their granddad a funeral pyre, set it alight, and float him out to sea in their version of a Viking sendoff.

The three kids go back to the house, where party preparations are in full swing, and tell their parents that Gordie died and what they did with his body. Gavin calls emergency services, and the police show up with an agent from Child Protection [Celia Imrie], who begins what she terms an "intervention" that threatens to end in Doug and Abi's children being removed from the home.

Meanwhile, as reporters swarm the house, Mickey and Jess find a Youtube clip of Gavin's wife aka their mentally ill Aunt Margaret [Ameila Bullmore] losing it in a supermarket, and broadcast it to the giant screen set up for Gordie's party.

After Mickey faces the reporters and tells them what he and his sibs did, Doug goes out to try to do damage control, but is bumbling and inarticulate. Right behind him is Abi, who staunchly defends Doug and the children's actions.

(Meanwhile Gavin's violinist son Kenneth, who up until then ignored Gordie's pleas to go wild, dutifully played only the serious music his father wants him to play, joins the band and falls for its female fiddler).

Finally, Lottie having used Gordie's words to chastise the adults for being such idiots, they all gather on the beach for a more fitting farewell to Gordie, after which Doug and Abi tell their kids that while they'll still be living separately, they've dropped their custody battles and will now be behaving like adults.