Holly (Katherine Heigl) and Messer (Josh Duhamel) are the best friends of married couple Peter and Alison (Hayes MacArthur and Christina Hendricks) but share a mutual dislike of each other. When Peter and Alison are killed in a car crash, Holly and Messer are named as legal guardians for their one-year-old daughter Sophie. As they take on the task of raising Sophie, they have to come to terms not only with living their new lives, but living with each other.
Having Heigl as co-lead is the film's main point of strength, being as she is a likeable and talented presence; unfortunately here she comes across as something of a one-note character thanks to the flat scripting and direction. This is further exacerbated by the lack of chemistry between her and Duhamel's character, who also provides another of the film's key problems. Messer comes across as entirely unlikeable for a sizeable chunk of the film. One of the first things we see him do - during the opening credits no less - is to pretend to drop newborn Sophie as a joke. At best a heavy-handed attempt to show Messer as a poor candidate for fatherhood; at worst, a cheap and tasteless joke which assumes that child neglect is amusing.
In fact, a lot of Life As We Know It's humour comes from this assumption, which produces a fair few uncomfortable misses. Thankfully there are a handful of sequences that are actually quite funny, such as one where Holly and Messer wind down by watching some of Sophie's kids' TV programmes after eating a batch of "special" brownies. But even these provide short-lived relief as the film continually swings sharply in tone, meaning any uplifting feeling is usually dampened down by something depressing happening.
The film trundles towards its inevitable conclusion, taking about half an hour too long to do so, with Holly and Messer helped along the way by the one-dimensional neighbours who crop up as a collective every so often, and what little character development there is feeling shoved in hamfistedly when the writer realised the story couldn't carry on if one of the characters didn't change in some way.
In essence, Life As We Know It doesn't make many more mistakes that your average rom com. The main issue is the extra elements that are thrown in - mourning young friends, unexpectedly raising someone else's child - and the fact that director Greg Berlanti really doesn't know what he wants to do with them in terms of tone or their function within the story. The result is a film that leaves you feeling just as unsure as to how you should take it, and that is ultimately underdeveloped and unsatisfying.