We first meet Adam at the age of 12 (Walker Scobell), finding out that he's very smart - but he's small, asthmatic, and can't keep his mouth shut, so he's bullied at school. Then we meet his Mom (Jennifer Garner) who's struggling to keep it together as a single mother after the fairly recent death of her husband, Adam's father. Then we meet Adam, age 40 (Ryan Reynolds) ... when he jumps into 12 year old Adam's life in a time jet from the future. And we find out that their father actually created the science that led to time travel ... and someone else abused it. There's snark, time jumps, and several slightly weird but fun fights.
Reynolds isn't doing anything too different: he brings the abundance of snark and sarcasm. But he brings good comedic timing and charm, and they found a young kid (Scobell) who's a very good younger version of him. The end product is a lot of fun.