That crazy little thing called love complicates a seemingly perfect relationship based on sex in Ivan Reitman’s contemporary comedy of social mores. As the title suggests, No Strings Attached centres on a pair of 20-something professionals who do not have time in their busy schedules for an inconvenience such as commitment.

And so they agree to be friends with benefits, calling and texting each other for physical gratification with the understanding that they do not have to worry about the awkwardness of post-coital conversation or sneaking out before breakfast. As Harry and Sally discovered, when they met more than 20 years ago in Rob Reiner’s iconic romantic comedy, men and women cannot be friends without emotions getting in the way.

Thus screenwriter Elizabeth Meriwether gradually shows her protagonists the errors of their lustful ways and sets up the possibility of a happy ever after using that age-old plot device: A family wedding.

In the wrong hands, No Strings Attached would be saccharine nonsense. Thankfully, this film is blessed with director Ivan Reitman and well-judged performances from Natalie Portman and Ashton Kutcher, who generate smouldering on-screen chemistry.

Adam (Ashton Kutcher), whose father Alvin (Kevin Kline) is a famous actor, works as a lowly TV producer on a show that is cynically jumping on the High School Musical bandwagon. After disastrous previous encounters, Adam stumbles back into the life of medical student Emma (Natalie Portman), who once told him, “People aren’t meant to be together.”

They agree that there would be no harm in no-strings-attached sex as and when the need arises. Adam’s friends Eli (Jake M Johnson) and Wallace (Chris ‘Ludacris’ Bridges) are envious that their buddy has found the perfect woman. Meanwhile, Emma’s housemates — including fellow medical student Patrice (Greta Gerwig) — diagnose imminent heartbreak.

This is the first touchingly sweet and uproariously funny Hollywood rom-com of the year, anchored by winning performances from Kutcher and Portman. Their comic timing is impeccable but more crucially, they both allow the characters to wear their hearts on their sleeves, shedding tears as they face the possibility of losing each other.

Husbands and boyfriends — take note.

I Am Number Four opens with a night-time chase through a jungle in Kenya, culminating in the grisly demise of a boy. “Number Three is dead,” gasps 15-year-old John Smith (Alex Pettyfer), who lives hundreds of miles away with his guardian Henri (Timothy Olyphant). John looks like any other high school student but he is really an alien from the doomed planet Lorien, hiding on Earth from a destructive rival race called the Mogadorians. These aggressive hunters intend to kill the Lorien refugees in a specific order, and since John is Number Four, he is next on their hit-list. When the Mogadorians finally catch up with John and Henri, they escape to the quaint town of Paradise, Ohio, where John befriends students Sam Goode (Callan McAuliffe) and Sarah Hart (Dianna Agron).

I Am Number Four maintains a brisk pace with regular set pieces, peppered with digital trickery and a smattering of bloodshed to justify the 12A certificate.

McAuliffe acts his co-stars off the screen as the sidekick who laments: “My entire childhood has been an episode of X-Files.”

Pettyfer copes with the physical rigours of his role and he makes an attractive pairing with Agron, who couldn’t be more adorably cute if she tried.