Think of one of your favourite childhood films. You maybe saw it in a cinema when you were small, possibly hyped up by a combination of excitement and sugar. Something in that film captured both your attention and your imagination. As soon as you could, you watched it on VHS or DVD, or recorded it off the television. Even today, as an adult, you can always go back and watch it over and over. The film is comfort food to you. You know the dialogue off by heart, but watching it again gives you a warm sense of nostalgia. The very best family films can do this for you, whether one’s personal favourite is The Wizard of Oz, or Dumbo, or The Dark Crystal, or The Lion King. The point of this preamble is this: over the next few weeks there are a lot of children who are going to be taken to see John Krasinski’s new film IF, and 20 years from now they are probably going to still be watching it. Some will even introduce it to their kids.

It is difficult in commercial cinema to express original ideas. With studios investing tens of millions – sometimes hundreds of millions – on each new feature film, remakes, sequels, and adaptations provide a known brand and a sense of safety. About as difficult a proposition is making the family film: that delicately constructed genre that appeals equally to young children and full-grown adults. It is easy to make it by throwing a few adult jokes or pop culture references into a children’s film, but that betrays the core appeal of the form. An original family film, such as the one Paramount Pictures have bankrolled and released here, is a rare and valuable thing. A genuinely great example of the form is rarer still.

Lest this review turns into a gushing list of superlatives, it is important to note that IF is not perfect. Its plot structure gets a little muddled in the middle, and a slightly overlong first act may test the patience of younger viewers, but overall, this is the exact sort of movie that families should make an effort to see together.

Bea (Cailey Fleming) lives with her grandmother (Fiona Shaw) while her father (John Krasinski) is confined to hospital. In an apartment upstairs she uncovers a community of imaginary friends – knowns as IFs – who have been separated from their children and long to find new ones. With the help of the cynical human guardian Cal (Ryan Reynolds), Bea sets up an agency to match IFs with children.

The IFs are outstanding. Each character is developed with its own distinct appearance and personality, including a giant purple monster, a 1920s-styled cartoon ballerina, even an animated glass of water with an ice cube for a face.

Krasinski has written and directed a beautiful film. It is high not just on laughs and imaginative designs, but on warmth and charm. It captures difficult subject matters in a smart and effective way. As a whole the film seems so delicately and carefully composed. In each individual moment or set piece it offers wonderful characters playing out emotionally effective stories. Every element is memorable; a raucous musical sequence mid-film is, I think, the highlight.

It is boosted by wonderful performances. Cailey Fleming showcases a great and emotional performance that remains surprisingly grounded throughout – the film would be unlikely to hit the beats it does without her at its centre. Krasinski and Fiona Shaw are engaging and memorable as Bea’s father and grandmother respectively. The all-star voice cast, including Steve Carell, Louis Gossett Jr, and Phoebe Waller-Bridge, are distinctive and charming.

I think the real surprise here is Ryan Reynolds, best known for his self-aware, somewhat lurid portrayals in more adult fare, presenting a character who is so sympathetic and subtle. It is easy to overlook actors in family films, and it would do Reynolds a disservice here to overlook the performance he provides.

There is heart here, and emotional honesty. There is resonance; one punctured regularly by sheer delight. IF is not simply a wonderful film. It represents a strong achievement in creativity and all-ages entertainment, and deserves to be a success. If it looks as if you might enjoy, I think you almost certainly shall.