Like most kids, James (Kyle Mooney) has a favourite TV show he's obsessed with. In his case, it's a bizarre science-fiction series called Brigsby Bear - which focuses on a heroic bear and his twin sidekicks the Smiles sisters, who battle to save the universe.

Despite the show's shoddy production values and ludicrous mythology (think early Doctor Who meets the 1990s version of the Power Rangers shot with the budget of a badly-made educational film), he spends hours watching Brigsby videos and chatting to other fans via an old computer.

James' situation is not all it seems though; he is not a kid, but in his mid-20s, and Brigsby Bear does not exist to anyone else - even as a fictional entity. This is because he's been raised in a desert bunker by Ted and April (Mark Hamill and Jane Adams), a couple who kidnapped him as an infant. To keep him unaware of his true origins they have constructed an elaborate web of lies with Brigsby Bear at its centre - telling their 'son' that they are restricted to their home and surviving on a hostile Earth.

When James is rescued by federal agents he discovers the truth - Ted has been filming all 35 volumes of Brigsby Bear, donning a giant homemade bear suit and casting an unwitting local waitress (Kate Lyn Sheil) as the Smiles sisters. James' fellow fans are figments of his captors' imagination, and his computer doesn't connect to the outside world.

As all this takes place in a truncated opening act, and the bulk of the film is about James' attempts to reintegrate into modern society and get to know his real mother (Transparent's Michaela Watkins), father (Veep's Matt Walsh) and teenage sister Aubrey (Ryan Simpkins).

Predictably, after living in isolation his whole life James finds this difficult, and clings to Brigsby as a connection to his old reality. In addition to providing a link with his past, his obsession with a TV show no one has ever heard of makes him something of a curiosity to his sister's friends as well as a bit of an Internet celebrity.

Distraught that there will be no conclusion to the Brigsby Bear story James and his sister's pal, Spencer (Jorge Lendeborg Jr.), decide to make a Brigsby Bear film. His birth parents object, but with the help of Detective Vogel (Greg Kinnear), a kind police officer assigned to the case, the duo obtain some of Ted's old props and costumes and begin work on their project in the hope of filling the voids in their lives.

There are plenty of laughs in director Dave McCary's first feature, a bizarre, if touching coming of age story. The tone is a little uneven however, as the film flits between fully embracing the absurdity of its premise and pulling at the heartstrings by covering traditional themes like youthful alienation.

Yet there is enough charm to carry audiences through, and its quirkiness is interesting rather than twee and grating. There's also an admirable attention to detail in a script that uses its unusual plot to lovingly examine the nature of geek culture and outsider status by taking it to its extremes. Mooney meanwhile is excellent in a difficult role that requires comic timing and emotional sensitivity.

McCary and the film's star both cut their teeth on Saturday Night Live, and unlike some of the sub-par projects alumni of the sketch comedy show have embarked upon, Brigsby Bear can be regarded as a qualified success - and is certainly worth the ticket price.