Keeping track of who’s who in this superhero multipack poses a challenge, but the screenplay packs a powerful punchWith his propaganda-poster jawline and air of unimpeachable nobility, Captain America (Chris Evans) has always been one of the duller members of the Marvel universe. Which is probably why, although this latest Avengers outing bears his name, the film-makers have press-ganged (almost) every spare ounce of Marvel muscle to back him up.